Ukraine's Secretary of the Security and Defense Council, Oleksiy Danilov, said that the Kyiv army has already destroyed 20 per cent of the Russian fleet in the Black Sea. Mr. Danilov, in an interview with the Ukrainian service of Voice of America, said that he wished that the prospects of the country's territory liberation would improve but emphasized that simultaneously with on-ground battles, crucial events occurred on the Black Sea.

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"Twenty percent of the fleet of the Russian Federation, including massive warships like the famous flagship of the Black Sea Fleet "Moscow," is missing. Last night's event confirms that our military understands the Crimea issue. Crimea is enormously significant for us, like the rest of our land."

Source:  Voice of America [Ukrainian service]

Which warships from its Black Sea navy did Russia also lose?

In April, Russia lost its flagship warship "Moscow".
In March, the Tapir-class landing ship Saratov was destroyed while in port in the occupied city of Berdiansk; a significant loss for the fleet according to the specialists.
The Kilo-class submarine Rostov-on-Don, was damaged beyond repair and Ropucha-class landing ship Minsk by Ukrainian cruise missiles in September during the attack on Sevastopol port.
In November, Russia lost newly constructed corvette, Askold.

What other Black Sea Fleet warships Russia had damaged or destroyed?

The losses include 1 Project 02510 BK-16E high-speed assault boat, 1 Project 640 small patrol boat and Vasily Bekh Project 22870 rescue tug. All ships were destroyed. The damaged warships include 1 Project 266M Natya-class minesweeper, and 1 Serna-class landing craft.