Kyiv claims to have destroyed a third of Russia's vessels in the Black Sea since Moscow launched its offensive over two years ago, a major embarrassment and setback for Russia's military even as its troops advance on the ground.

Russian Defence Minister Sergei Shoigu said President Vladimir Putin had issued a decree appointing vice-admiral Sergei Pinchuk as commander of the Black Sea Fleet.

The appointment comes as part of a wider shake-up of Russia's navy, including the appointment of a new overall commander-in-chief, admiral Alexander Moiseyev.

"On one hand, I congratulate you, on the other I hope you understand the enormous responsibility that lies on your shoulders, and I hope that you will deal with the tasks which will be put before you," Shoigu told the new fleet chiefs in a video message published by the defence ministry.

Russian military bloggers, some with close connections to the armed forces, had reported for weeks that the previous commander of the Black Sea Fleet, Viktor Sokolov, had been ousted amid the spate of ship attacks.

Moiseyev's appointment as navy chief was confirmed by state media last month.

Pinchuk, a 52-year-old career navy officer, was born in Sevastopol, the historic home of the Black Sea Fleet on the peninsula of Crimea, which Moscow annexed from Ukraine in 2014.

Satellite images show that Russia has moved many ships further east to a base at Novorossiysk, in a bid to better protect them from Ukrainian attacks.

In the most recent strike, on March 24, Ukraine claimed to have hit two large landing ships and a communications centre in a missile strike on Sevastopol.

Kyiv has previously hit ships in the Black Sea in nighttime attacks using naval drones packed with explosives.

The changes to Russia's navy leadership are the biggest shake-up of Russia's top military brass since the sacking of Sergei Surovikin, the overall chief of Russia's air and space forces, last year.