Briefing reporters on condition of anonymity, the official confirmed Kyiv's account of the incident -- which Russia said was caused by exploding ammunition on board.

"We assess that they hit it with two Neptunes," the official said, referring to Ukrainian anti-ship cruise missiles.

He said the strikes were believed to have caused casualties, but that it was "difficult to assess how many," adding that the United States had observed survivors being recovered by other Russian vessels in the area.

Russia has said the Moskva's crew was evacuated to nearby ships.

The missile cruiser had been leading Russia's naval effort in the seven-week conflict in Ukraine, playing a central role in the siege of the port city of Mariupol.

It sank Thursday after an explosion and fire that Ukraine claimed was caused by a missile strike -- while Russia said damage caused by exploding ammunition had caused the ship to "lose its balance" as it was being towed to port.

"It's a big blow symbolically," the Pentagon official said. "There is a pride aspect."

A knock-on effect of the sinking, he predicted, could be a form of "risk aversion" in the Russian navy.

But even more, he said, losing the Moskva -- one of just three Slava-class cruisers in Moscow's fleet -- creates a "capability gap" for the Russian navy in southern Ukraine.

Under the Montreux convention, he explained, Turkey keeps the Bosphorus and Dardanelles straits closed to warships in times of conflict, meaning Moscow cannot dispatch a replacement for the Moskva to the Black Sea.