Four civilians were reported killed in an explosion on a Crimean beach on Sunday. Russia says that Ukraine launched a U.S.-provided ATACMS ballistic missile at the beachgoers.

Russia is using the deaths to increase fears of a possible U.S.-Russian conflagration. Vladimir Putin’s spokesman Dmitry Peskov warned on Monday that the “involvement of the United States, the direct involvement, as a result of which Russian civilians are killed, cannot be without consequences … time will tell what these will be.” The Kremlin’s implication is that Russia will take some kind of direct response against the United States, potentially risking a broader conflict between the two nuclear powers.