One of Russia’s biggest oil tankers was struck by a maritime drone, the latest salvo in a Ukrainian military campaign employing unmanned vehicles to attack far-away Russian targets by air and by sea.

The Russian-flagged ship, the Sig, was hit by a drone carrying 450 kilograms (992 pounds) of TNT shortly before midnight, according to a Ukraine Security Service source. The strike created a hole in the vessel’s engine room at the waterline on the starboard side, forcing the 11-strong crew to fight the water intake, Russian authorities said. The flooding eventually stopped.

Russia’s Federal Agency for Marine and River Transport said no casualties were reported and that the Sig was not carrying oil when the drone crashed into ship. Ukrainian officials, however, said some crew were injured and that the tanker was carrying fuel for the Russian military.

CNN has not been able to independently verify the claims.

The strike on the Sig came just hours after Ukrainian sea drones targeted a major naval base in Novorossiysk, a coastal city on the Black Sea that is home to Russia’s largest port by volume of cargo handled. An amphibious Russian landing ship was hit, leaving it tilting badly and sitting very low in the water.

Ukraine has stepped up its attacks using unmanned aerial vehicles in recent weeks, hitting targets well within Russian territory, including in Moscow.

The new generation of powerful sea drones, however, could open up a new front for Kyiv in the 18-month conflict.

The weapons are fast, semi-submersible drones, and are proving very difficult to defend against. They can be easily launched at sea and at least some variants are capable of traveling several hundred miles to their targets.

Their payloads so far have proven capable of crippling large vessels.