Britain's defense ministry says Russia's Black Sea Fleet maintains attack capabilities after an attack on its command headquarters in Crimea that Ukraine says killed the fleet's commander.

The Ukrainian military said on Monday that its forces struck the command headquarters in the port city of Sevastopol on Friday. They claimed that 34 senior Russian naval officers, including the top commander, died in the attack.

Moscow has made no official comments, but Russian media outlets quoted the country's defense ministry sources as saying the Ukrainian allegation is false.

The British defense ministry said in its intelligence update on Tuesday that the Black Sea Fleet has suffered a series of major attacks in recent weeks.

However, the ministry said the physical damage to the fleet is "almost certainly severe but localised." It said, "The fleet remains capable of fulfilling its core wartime missions of cruise missile strikes and local security patrols."

The ministry added that Ukraine's "deep strike battle" in the Black Sea is "likely forcing Russia into a reactive posture," suggesting concern about a probable retaliation by Moscow against these attacks.

US think tank Institute for the Study of War, said on Monday that the deaths of the top admiral and other high-ranking Russian commanders have yet to be confirmed.

But it says if confirmed, their deaths "would create significant disruptions in command and control" of the fleet.