The guided missile cruiser sank in rough seas after being damaged by fire during Russia's military operation in Ukraine, Moscow's defense ministry said.

But Ukraine said the ship had been hit in a rocket attack.

"This is a big blow to the Black Sea fleet, this is... a key part of their efforts to execute some sort of naval dominance in the Black Sea," Pentagon spokesman John Kirby told CNN.

"This is going to have an effect on their capabilities."

Kirby said the United States could not confirm the exact cause of the ship's demise.

"We're also not in any position to refute the Ukrainian side of this," he added. "It's certainly plausible and possible that they did in fact hit this with a Neptune missile or maybe more."