Ukrainianunderwater drones have struck two tankers belonging to Russia's so-called shadow fleet in the Black Sea, according to an official in Ukraine's security services (SBU).
Ukraineclaimed the attacks after explosions hit the two vessels on Friday and Saturday.
A Ukrainian security source told CNN that Sea Baby maritime drones had been used in a joint operation by the SBU and the navy. There was no immediate comment from Russia.
Both tankers sustained critical damage, the source said, and were effectively taken out of service. "This will deal a significant blow to the transportation of Russian oil."
Russia uses hundreds of tankers – many sailing under different flags of convenience – to ship its oil to customers in defiance of sanctions.
The Gambian-flagged tanker Virat was attacked again on Saturday, after first sustaining damage on Friday, according to Turkey's Transport Ministry.
Minor damage was sustained above the waterline, according to Turkish maritime authorities, and there was no fire. The vessel is some 30 miles (50 kilometers) off the Turkish coast. Ship tracking data showed it slowing and turning towards the coast late on Friday.
"There is no request from the personnel to abandon the ship," the Turkish Transport ministry said, but a fire-fighting tugboat had been sent to the scene.
The Turkish foreign ministry expressed concern over the attacks, which "posed serious risks to the safety of navigation, life, property, and the environment in the region."
The Virat's destination was unclear. Shipping data showed it awaiting orders in the Black Sea. It was sanctioned by the United States in January when it sailed under a different name, and by the United Kingdom and European Union later this year.
There was also an explosion on another ship that carries Russian crude oil in a nearby part of the Black Sea on Friday. All 25 crew members on board that tanker, the Gambian- flagged Kairos, were evacuated. Neither vessel was in Turkish waters when they were hit.
Video showed Turkish tugs tackling an extensive fire on board the ship about 30 miles off the Turkish coast. On Saturday the Turkish Transport ministry said that a fire on the open deck of the Kairos had been extinguished.
The Kairos is 275 meters long and weighs nearly 80,000 tons. It was sanctioned by the EU earlier this year and left an Indian port earlier this month to return to the Russian Black Sea port of Novorossiysk.
Earlier this year, several unexplained explosions occurred on ships that have carried oil for Russia from the Black Sea.
Both the Virat and the Kairos had transited the Bosphorus Strait into the Black Sea. Other ships also sanctioned for carrying Russian crude oil were traveling the same route, according to shipping data on Saturday.
There was also an attack by a marine drone early Saturday on a mooring point at the Russian port of Novorossiysk in the Black Sea, according to the Caspian Pipeline Consortium.
The pipeline carries oil from Kazakhstan – through Russia – to customers overseas. Kazakhstan's Energy ministry said it had "activated a plan to redirect export oil volumes to alternative routes" after the strike.
Novorossiysk has been attacked by Ukrainian drones multiple times. CNN has reached out to Ukraine's security services for comment.
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