A Ukrainian sea drone full of explosives struck a Russian fuel tanker overnight near a bridge linking Russia to annexed Crimea, the second such attack in 24 hours, both sides said on Saturday.

No one was hurt, but the Crimean Bridge and ferry transport were suspended for several hours, according to Russian-installed officials in Crimea, which Moscow seized from Ukraine in 2014.

A Ukrainian intelligence source told Reuters that the drone with 450 kg of explosives hit the SIG vessel as it transported fuel for the Russian military in Ukrainian territorial waters.

“The tanker was well loaded with fuel, so the ‘fireworks’ were seen from afar,” the source said, of the joint operation by Ukraine’s navy and security service.

Kyiv says destroying Russia’s military infrastructure inside Russia or on Russian-controlled territory in Ukraine is crucial to its counteroffensive after the February 2022 invasion.

Another sea drone attack on Russia’s navy base at Novorossiysk damaged a warship on Friday, the first time the Ukrainian navy had projected its power so far from its shores.

And a Ukrainian government agency warned on Saturday that six Russian Black Sea ports - Anapa, Novorossiysk, Gelendzhik, Tuapse, Sochi, and Taman - were in “war risk area”.

Moscow has bombed Ukrainian ports since its withdrawal last month from a U.N.-brokered deal to let Ukraine export grain.

Fuel for troops

The SIG tanker had been supplying oil to Russian troops in Syria, according to Vladimir Rogov, a Russian-appointed official in Ukraine’s southeastern region of Zaporizhzhia.

The United States imposed sanctions on the tanker and its owner, St. Petersburg-based Transpetrochart, in 2019 for helping provide jet fuel in Syria.

Vasyl Malyuk, head of Ukraine’s SBU security service, did not directly confirm the latest attack but said any incident with Russian ships or the Crimean bridge was “an absolutely logical and efficient step towards the enemy”.

“Moreover, such special operations are conducted in the territorial waters of Ukraine and are completely legal,” Malyuk said on the Telegram messaging app.

Russia’s Novorossiysk Maritime Rescue Coordination Centre was quoted by the RIA news agency as saying there was no fuel spill from the SIG, as the ship had been carrying only technical ballast. Recovery work was underway with two tugboats nearby.

Rogov posted an audio clip on Telegram in which the SIG requested a tow. He also posted pictures of what he described as shattered fixtures and equipment inside the vessel.

“The SIG tanker ... received a hole in the engine room near the waterline on the starboard side, preliminarily as a result of a sea drone attack,” Russia’s Federal Marine and River Transport agency said in a statement on Telegram.

The Moscow-installed authorities in Crimea said the bridge, which was completed by Russia in 2018 and has come under serious attack twice in the war, was not targeted.