A drone attack on the south of the Black Sea peninsula of Crimea, a part of Ukraine which was annexed by Russia in 2014.
It might have hit and sunk a Russian landing ship, according to Ukrainian media.
The Ukrainian media outlet, Ukrainska Pravda cited an anonymous source at the military intelligence service HUR as saying that the Russian Black Sea fleet had been attacked with drones.
The news site reported that the Russian landing ship Caesar Kunikov had been so badly damaged that it had sunk. This could not be verified independently.
There was initially no official statement from Ukraine.
The Russian Defence Ministry said only that six Ukrainian drones were shot down over the Black Sea during the night.
Ukraine had been defending itself against a full-scale Russian military invasion for almost two years.
The Ukrainian military had repeatedly sunk or at least severely damaged Russian warships with missiles and naval drones.