Russia said a day earlier it had sunk a Ukranian reconnaissance ship, the Simferopol, in the Danube river delta in one of its first successful hits with a naval drone.

Ukraine's admission that a ship was hit is rare, as both Moscow and Kyiv are reluctant to disclose military losses.

"We are still searching for several soldiers," who were on the targeted ship, navy spokesperson Dmytro Pletenchuk told AFP. He declined to confirm the vessel had been hit by a drone or where the attack took place.

Others were wounded in the attack, Pletenchuk said.

Ukraine, which had a much smaller maritime presence than Russia before the invasion, has deployed naval drones to significant effect throughout the war since Russia's 2022 invasion.

But for Russia they are a relatively new innovation. Its defence ministry published Thursday black-and-white footage of the claimed attack, which showed a large explosion on the water.

A string of successful Ukrainian naval drone attacks forced Russia to move its fleet out of the western part of the Black Sea earlier in the conflict.

Kyiv has also been able to open an export corridor along its southern coast to enable vital sales of agricultural goods.