Ukraine has used long-range drones to attack Russian oil platforms as it expands its strikes on the country's energy sector.

Ukraine claimed it carried out an underwater drone strike on Dec. 15, the first such attack in maritime warfare, that critically damaged a Russian Kilo-class attack submarine at the Russian port of Novorossiysk,

The incident follows Turkey’s warning last week of Black Sea escalation after Russian attacks on Ukrainian ports that damaged three Turkish-owned cargo vessels. In a statement, the ministry says Turkish and NATO F-16 jets were put on alert to ensure the security of Turkish airspace after the detection of the drone.

The Russian submarine was allegedly carrying Kalibr cruise missiles, which Russia has used in devastating strikes on critical infrastructure and military targets across Ukraine. The SBU said the submarine was “put out of action” as a result of the blast. The FT could not independently verify the claim.

The drones are still in limited supply and are not always the ideal interception method, Ukrainian soldiers and an official told Business Insider.

At least two people have been killed in a drone attack in Russia’s Saratov region, according to local authorities.