I read today that in the Gulf of Oman, in international waters, a small Iranian ship was detected by the US Navy. The Americans assumed that it transported weapons to Yemen and contacted the British Royal Navy, which sent a frigate to intercept the Iranian boat and confiscate the weapons on board, on the grounds that they were illegal. I wonder what the legality of this action is.
The United Nations Security Council made Resolution 2216 on 14 April 2015, and reaffirmed the key provisions recently in Resolution 2675 of 15 February 2023 (see the reference in paragraph 1). Resolution 2216, in paragraphs 14-17, imposes an arms embargo for Yemen. In particular, it calls on all Member States to "take the necessary measures to prevent the direct or indirect supply, sale or transfer" of armaments, and to inspect Yemen-bound cargo for the same, "in accordance with ... the law of the sea" and other relevant law.
The UK and USA are both Member States of the UN, so they are specifically empowered to implement the embargo.
The reason the Security Council can do this is because the UN Charter says that it can. "Interruption of economic relations" and "blockade" are among the possibilities explicitly mentioned (Articles 42 and 43), but this is well within the scope of the Security Council's power.
By saying that interdiction should be in accordance with the law of the sea, the UNSC is bringing in well-established rules about the conduct of maritime operations, jurisdiction in the high seas and elsewhere, compensation for wrongful interdiction, use of force, and so on. In other words, they are not trying to override the standard rules. This is not the first embargo in history and there is no need to outline everything in the resolution text.