The US is concerned Russia may supply missiles to the Houthis in Yemen, an American official said, a move that could stoke further Middle East unrest by strengthening one of the militant groups fighting Israel.

The strike on power stations and a seaport, which killed four people, was the second known time that Israel has retaliated against the militant group’ attacks by striking in Yemen.

Tensions in the Middle East have further increased following the assassination of Hezbollah leader Hassan Nasrala on Friday in an Israeli bombing in the southern suburbs of Beirut.