Russia's Mosvka warship was hit by two Ukrainian missiles before it sank in the Black Sea, a senior Pentagon official said Friday on condition of anonymity.

The senior official said these were Neptune anti-ship missiles. The U.S. believes there were Russian casualties, though numbers are unclear, the official added.

Russian news agencies cited the Russian Defense Ministry on Thursday as saying that the missile cruiser Moskva sunk in stormy seas after what it said was a fire and explosions involving ammunition stowed onboard.

Ukraine said it launched a missile strike on the Moskva from the coast, claiming it ripped open the Soviet-era ship.

However, Reuters was unable to verify either side's assertions.