The Russian foreign ministry summoned the American ambassador on Monday to protest what it says was the use of US-made advanced missiles in a Ukrainian attack on Russian-annexed Crimea that reportedly killed four people and wounded more than 150.

Washington “has effectively become a party” to the war on Ukraine’s side, the ministry said in a statement, adding, “Retaliatory measures will certainly follow.” It did not elaborate. There was no immediate comment from US or Ukrainian officials. The Associated Press could not independently verify Russia’s claims about the missiles used.

Kyiv’s forces have relied heavily on western-supplied weaponry since Russia’s invasion more than three years ago. The military aid has been crucial in allowing Ukraine to hold the Kremlin’s army at bay.

Russian authorities said that the dead in Sunday’s attack included two children who were hit by falling debris from Ukrainian missiles that were shot down over a coastal area in Sevastopol, a port city in Crimea.

Russia said the missiles were US-made ATACMS, a long-range, guided missile.