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.

The Israeli Air Force launched its largest attack against the Houthis in Yemen so far, bombing power plants and a seaport. Al-Masirah, the Houthi-run news station, said that at least four people were killed — one port worker and three engineers — and 33 injured.