Ukraine's special forces said on Friday they had conducted an operation alongside what they described as a local resistance movement to hit two Russian ships transporting weapons and military equipment in the Caspian sea.

They did not specify when the strike took place. A Ukrainian official said on Thursday that Kyiv's drones had hit a Russian oil rig in the Caspian Sea for the first time, disabling the extraction of oil and gas from about 20 wells.

The special forces' statement on Telegram did not say how they had hit the vessels or what the extent of any damage was. They said the ships were hit off the coast of the republic of Kalmykia, a region of Russia.

They named the vessels as the Composer Rakhmaninoff and the Askar-Sarydzha, which they said were sanctioned by the U.S. for carrying military cargoes between Iran and Russia. The statement said that the "Black Spark" resistance movement had provided detailed information on the movement and cargo of the ships.