The Closure of the Strait of Hormuz by Iran: Considerations over the Law of the Sea and Law of Naval Warfare

[Konstantinos Deligiannis-Virvos is a PhD Research Fellow with the Norwegian Centre for the Law of the Sea (NCLOS) at the Law Faculty of UiT-The Arctic University of Norway]

The illegal armed attack against Iran by Israel and the US on February 28, 2026 prompted Iran to “close” the strait of Hormuz to international navigation. The attack against Iran clearly contravenes Art. 2(4) UN Charter but whether the response of Iran regarding the closure of the strait of Hormuz has been in accordance with international law warrants more careful consideration. The threats against international shipping accompanying the closing of the Strait of Hormuz that facilitates the transfer of approximately one-fifth of the oil consumed globally, as well as large quantities of LNG and fertilizers, has created a dire situation to the shipping industry and to the global economy. This post briefly investigates the legal rules pertaining to the closure and Iran’s conduct in respect to maritime activities in the area as well as the legality over attacks on vessels navigating within and beyond the strait of Hormuz.

The Strait of Hormuz Under the Law of the Sea 

The strait of Hormuz is bordered by Iran and Oman and measures approximately 20¾ nm in width at its narrowest point where the territorial seas of Iran and Oman overlap. The strait is at its deepest close to the Iranian coast. It connects the EEZs of the States bordering the Persian Gulf and the Gulf of Oman. It is also an extremely busy shipping corridor. It thus meets the geographical and functional criteria to be characterized as a strait used for international navigation, as per the Corfu Channel Case, p. 28-29 and Art. 37 UNCLOS (Lott, 161-162). However, whether the regime of transit passage under article 38 UNCLOS applies to the strait of Hormuz is a contentious issue especially for the current situation, since Iran, Israel and the US are not parties to the UNCLOS. 

The right of transit passage for straits used for international navigation was first introduced in UNCLOS and its status as a rule of customary international law is contested. The US maintains that the regime of transit passage is part of customary international law and thus it applies to the strait of Hormuz (see Lott, 12 and Gioia, 31). On the contrary, Iran considers that the right of transit passage is not part of customary international law. Iran has consistently stated that the strait of Hormuz is subject to the regime of (non-suspendable) innocent passage. If the regime of transit passage is not part of customary international law, as Iran posits, then Iran can take the necessary steps to prevent passage that is not innocent (Art. 25 (1) of UNCLOS). But, even if the right of transit passage forms part of customary international law, it can also be argued that Iran has been a persistent objector with consistent practice against the formation of the rule. Specifically, Iran had expressed its disagreement with the regime of transit passage during the negotiations of UNCLOS (see Bagheri, p. 85-87, Kraksa, p. 350-352, and Van Dyke, p. 43), with its interpretative declaration upon signature of UNCLOS and its 1993 Act on the Marine Areas of the Islamic Republic of Iran in the Persian Gulf and the Oman Sea which refers only to the right of innocent passage (see Bagheri, p. 87-91). Thus, non-parties to UNCLOS, such as the US and Israel can only invoke the customary right of non-suspendable innocent passage through the strait of Hormuz as codified in Art. 45 UNCLOS (see Lott at p. 166-168 and recently who further supports that parties to the UNCLOS are still able to invoke the applicability of the right of transit passage by virtue of the fact that Iran is a signatory to the UNCLOS and cannot defeat its object and purpose). 

Naval Warfare in the Strait of Hormuz 

The strait of Hormuz is no stranger to international tensions and conflicts. Indicatively, in 2019 oil tankers struck mines that were laid at the approaches of the strait. Later on the same year Iran arrested two oil tankers, the UK flagged Stena Impero and the Liberian-flagged Mesdar. Even before that, during the Iraq-Iran war of 1980-1988, tankers sailing through the strait of Hormuz were repeatedly attacked by both belligerents, a situation that prompted the US to attack Iranian oil platforms, an issue that was eventually adjudicated by the ICJ.

The situation commenced in February 2026 undoubtedly constitutes an international armed conflict (IAC).  In an IAC at sea the matters not governed by the relevant UNCLOS provisions are governed by the, mostly customary, law of naval warfare which is expressed within the San Remo Manual on International Law Applicable to Armed Conflicts at Sea. The UNCLOS continues to apply in respect to navigational rights in various maritime zones.

According to the relevant customary rules, belligerents have the right to transit through neutral straits used for international navigation but when one of the belligerents is bordering a strait, this area may become an area of military operations where “hostile actions by naval forces may be conducted in, on or over it” (San Remo Manual Rule 10, Ronzitti, 6). Naval hostilities can be undertaken in the territorial seas of the belligerents, their EEZ and continental shelf, as well as on the EEZ of neutral States and on the high seas with due regard to the rights of neutral States (San Remo Manual Rules 14-37). International practice in relation to straits is consistent in that the belligerent State may close the strait to transit by enemy vessels but must respect the transit rights of neutral vessels (San Remo Manual Rule 27, Caminos and Cogliati-Bantz, 20-21 and 24-29 also Newport Manual, 74). Restrictions to passage might be implemented if they relate to aversion of danger and might include the exercising of visit and search rights by belligerent warships (Ronzitti, 6 and Caminos and Cogliati-Bantz, 26). However, a total closure by means of laying mines without providing for safe corridors would be considered illegal since it would not respect the navigational rights of neutral vessels. For the time being, it seems that this is not the case since a limited number of vessels have been permitted to exit the strait.

Limited State practice (supported by the US) contends that maritime operational zones can be established for a variety of reasons, including reducing the presence of merchant shipping in a particular area. Such zones must follow notification through appropriate channels, sufficient enforcement to achieve the zone’s purpose and balance the military requirement for the zone against any impact it has on neutral navigation (Newport Manual, 113-115). Absent the proper notification and the extensive impact on neutral navigation, the closure of the Strait of Hormuz cannot be regarded as a maritime operational zone.

Consequently, it is well established that Iran is able to close the strait of Hormuz to enemy vessels, however, it must respect the navigational rights of neutral vessels. Thus, threatening to attack any vessel that passes through the strait irrespective of which flag it flies cannot be considered legal under customary international law. However, conduct against merchant vessels is permitted in specific situations. 

Attacks Against Merchant Vessels in and Beyond the Strait of Hormuz

As the Newport Manual on the Law of Naval Warfare characteristically notes “the law of naval warfare is characterized by an element of economic warfare”. Beyond the rights of visit and search against neutral vessels, the belligerents might take certain actions against enemy and neutral vessels that are strictly regulated by international law. 

In the hours following the attack against Iran, at least three tankers were struck by missiles or drones. On March 2, 2026 the US flagged oil tanker Stena Imperative was hit while at port in Bahrain resulting in the killing of one shipyard worker and injury of two others.

Art. 51 and 52 of the Additional Protocol I to the Geneva Conventions regarding the general protection of civilians and civilian objects applies to the law of naval warfare. According to the San Remo Manual Rules 40-41, the merchant vessels including the tankers flying the US flag are non-combatants and cannot be attacked unless they constitute military objectives in accordance with Rules 40, 59, 60 and 67. Military objectives are limited to “those objects which by their nature, location, purpose or use make an effective contribution to military action and whose total or partial destruction, capture or neutralization, in the circumstances ruling at the time, offers a definite military advantage” (Art 52(2) AP I and San Remo Manual Rule 40). 

The situation over the Stena Imperative might be one of a legitimate military objective. The Stena Imperative is a participant to the Tanker Security Programme which provides the US Department of Defense with 10 oil tankers that “may be used to supply the armed forces with fuel during times of armed conflict or national emergency”. The Stena Imperative and its crew has been trained and certified by the US Military Sealift Command (MSC) to conduct underway replenishment operations for US naval forces. As part of the Tanker Security Programme and taking into account its nature and location at the time it can be argued that the Stena Imperative falls under the categories of vessels that may be attacked under Art. 52(2) AP I and the San Remo Manual Rule 60 (b) and (g). The anticipated military advantage would be, presumably, to hamper the military operations against Iran. At the time of the attack the Stena Imperative was anchored in a port of Bahrain. The legality of the attacks of Iran against the port as a US base can be contested and it depends on whether it can be assumed that those bases have been used to attack Iran. The details about the attack against the Stena Imperative do not allow for a definite answer at the moment. 

In any case however, such attacks need to follow the principle of precautions in attack (Art. 57 AP I). While incidental damage to civilians is often unavoidable, Art. 57 (2) AP I requires the attacker to verify the objectives to be attacked are military objectives within the meaning of Art. 52 (2) AP I, take all feasible precautions in the choice of means and methods of attack in order to minimize civilian casualties and injuries, and refrain from deciding to launch an attack that “may be expected to cause” civilian casualties or injuries which “would be excessive in relation to the concrete and direct military advantage anticipated”. While in the present case the first requirement was probably met, it can be argued that Iran could take the necessary precaution to protect civilian life and health and wait to attack the Stena Imperative after it left port to conduct underway replenishment operations. Thus, it can be assumed that the killing and injuries of the shipyard workers are prima facie against the relevant rules of the Additional Protocol I regarding the principle of precaution.

A question of particular importance for the present case regarding other oil tankers is whether they are contributing to the enemy’s “war sustaining effort” and whether this renders them military objectives. According to the Newport Manual, State practice regarding this issue is divided. It is my view that while the oil extracted and transferred through the Strait of Hormuz fuels the US and Israeli military, it cannot be said that any oil tanker sailing in the area irrespective of which flag it flies constitutes a valid military objective in accordance with Art. 52(2) AP I and the San Remo Manual, which expressively rejected the notion. The Newport Manual provides some interesting insights: it notes that the US doctrine supports this view but 

[a] majority of States do not recognize that a war-sustaining contribution to the war effort is within the definition of military objectives. According to the majority view, export products carried by neutral vessels cannot be considered military objectives and may be interfered with only by establishing a lawful blockade. Enemy merchant vessels and their cargo, however, are always subject to capture outside neutral waters for supporting the warfighting (or war-sustaining) effort, regardless of whether a blockade has been established. (p. 138-139). 

Indeed, there seems to be consistent practice expressed in San Remo Manual Rule 135 that “[…] enemy vessels, whether merchant or otherwise, and goods on board such vessels may be captured outside neutral waters. Prior exercise of visit and search is not required.” However neutral vessels cannot be captured unless it is proven that they undertake the actions stipulated in San Remo Manual Rules 67 and 146. Goods on board neutral merchant vessels are subject to capture only if they are contraband.

Moreover, it must be noted that merchant ships, whether enemy or neutral, may be attacked if they are sailing under an enemy convoy, escorted by warships or military aircraft (San Remo Manual Rules 60 (d) and 67 (e)). Thus the Trump assertion that the US will “protect ships in Middle East if necessary” while providing for an operational deterrent to the armed forces in Iran may also provide a legal basis for attacking those vessels. As Bamnios notes, whether an attack on such a convoy can be considered legal depends on the ‘nationality’ of the convoy which is to be “determined by the flag of the escorting warships”, but also the flag of the merchant ships. He further concludes that “the only legally waterproof combination of merchant vessels and warships that would form a convoy, legally protected from Iranian actions, would be one in which both the merchant vessels and the warships are of a neutral flag State.” At the time of writing no convoys have been formed in order to escort merchant vessels out of the strait of Hormuz.

Concluding Remarks 

The above brief analysis showcases that Iran is legally entitled to close the Strait of Hormuz for US and Israeli vessels but is under the obligation to not hamper the navigational rights of neutral vessels. Simultaneously, attacking merchant vessels, whether enemy or neutral, is not in accordance with customary international law, but Iran is still able to capture enemy vessels as prize or neutral vessels under strict circumstances (as is allowed to the US and Israel as well). Whether the attack against the Stena Imperative was legal can be contested. A final consideration that warrants discussion is whether the actions of Iran can be considered as countermeasures on the basis of Article 49-54 of the ILC Articles on State Responsibility (ARSIWA) but due to space constraints this must be addressed at a later time. It must nevertheless be underscored that, as stated in Art. 50(1)(a) ARSIWA, countermeasures shall not affect the obligation to refrain from the threat or use of force and obligations for the protection of fundamental human rights and obligations of a humanitarian character prohibiting reprisals.

Photo attribution: Photo by Abolhassan Neghabi on Unsplash