Once and then I read news about US navy stopping, searching and even sinking commercial vessels flowing non-US ensign (most recently Iranian). Sometimes news regard navy vessels not conforming to COLREG and threatening other vessels with deadly force to give way instead.

I know that navy (US Navy especially) are guys with big guns and Iran is considered bad guys by many, but what are the (international) legal grounds of such conduct?

I know that some misconduct (like piracy) falls under universal jurisdiction, which enable any navy to intervene, but cases I have read about regarded good drug/fuel "trafficking". Is not a naval blockade an act of war?

For the sake of answering the question assume that:

  • the state of ensign of the vessel is not a party of any international agreement that grants jurisdiction/authority to navy of any other state in case of the "misconduct",
  • the "misconduct" is either legal in the state of ensign, or the state is not interested in its prosecution.
  • Can you give any specifics on the incidents you are talking about? Where did they occur? When? What were the circumstances? What explanation if any was given by the US? Commented Nov 15, 2022 at 15:45
  • @NateEldredge I do not remember details of previous incidents, but I have googled for the last one in English. While what I read in Polish claimed the vessel to be Iranian, most sources do not mention its ensign and nbcnews.com/news/world/… claims it to be stateless which IMO makes the boarding legit.
    – abukaj
    Commented Nov 17, 2022 at 5:21

1 Answer 1


The limit on boarding is contained in Article 110 of the UN Convention on the Law of the Sea

Article 110

Right of visit

  1. Except where acts of interference derive from powers conferred by treaty, a warship which encounters on the high seas a foreign ship, other than a ship entitled to complete immunity in accordance with articles 95 and 96, is not justified in boarding it unless there is reasonable ground for suspecting that:

(a) the ship is engaged in piracy;

(b) the ship is engaged in the slave trade;

(c) the ship is engaged in unauthorized broadcasting and the flag State of the warship has jurisdiction under article 109;

(d) the ship is without nationality; or

(e) though flying a foreign flag or refusing to show its flag, the ship is, in reality, of the same nationality as the warship.

  1. In the cases provided for in paragraph 1, the warship may proceed to verify the ship's right to fly its flag. To this end, it may send a boat under the command of an officer to the suspected ship. If suspicion remains after the documents have been checked, it may proceed to a further examination on board the ship, which must be carried out with all possible consideration.

  2. If the suspicions prove to be unfounded, and provided that the ship boarded has not committed any act justifying them, it shall be compensated for any loss or damage that may have been sustained.

  3. These provisions apply mutatis mutandis to military aircraft.

  4. These provisions also apply to any other duly authorized ships or aircraft clearly marked and identifiable as being on government service.

COLREG requires that all vessels avoid collisions; there is no right-of-way in navigation. A warship is generally more maneuverable than a merchant vessel and therefore has a greater ability (and responsibility) to avoid collisions. However, a warship executing a war-like function against a non-complying vessel also has more leeway if a collision occurs.

Of course, Article 95 of UNCLA gives a warship complete immunity other than in their home state for acts on the High Seas, so an Iranian merchant ship that wished to pursue damages would need to do so in the US Admiralty Courts.

  • If I remember correctly what I have been taught at sailing course, COLREG is also enforced by the state of ensign, isn't it? AFAIR the US vessels used shot across the bow to enforce its right of way (and that made me curious whether warships are exempted from COLREG).
    – abukaj
    Commented Nov 17, 2022 at 5:56
  • I disagree with the linked article's assessment that the term "right of way" implies total absolution from any fault. Every automobile, nautical and aviation training course I have been part of simply requires that the non-right of way vessel has primary responsibility to yield to the other and initiate avoidance maneuvers. I have even seen it written in the law that vehicles having right of way that can avoid a collision yet fail to do so share fault with the other vehicle. Commented Dec 21, 2022 at 19:47

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