Russia’s government has lodged a complaint with the International Maritime Organization that accuses Ukraine of a “terrorist attack” targetting what it described as a civilian tanker in the Black Sea.

The complaint came just days after an aide to Ukrainian President Volodymyr Zelenskyy accused Russia of violating international law by firing warning shots at a general cargo ship in the Black Sea, as tensions continue to boil in the 18-month-old war’s maritime theatre.

Moscow’s IMO delegation said it “strongly condemns” the sea drone attack on the 6,619-dwt product tanker Sig (built 2014) on the night of 4 August.

“This reckless act posed a threat not only to the life of the crew, but also could have led to a large-scale ecological catastrophe,” the Russian delegation said.

Shortly after the Sig attack, Ukraine Security Service head Vasyl Malyuk described such operations as “completely legal” and a logical step in the face of Russian aggression.

Though Malyuk did not confirm Ukraine’s involvement at the time, Russia’s IMO delegation couched his comments as effectively taking responsibility for the strike.

“Such barbaric actions directed against civilian vessels and its unarmed crew cannot be justified or be left without a reaction,” Russia wrote.

While the Sig is a merchant vessel, Ukrainian officials have reportedly alleged that it was carrying fuel for Russian troops. US officials had previously sanctioned the ship and its operator, St Petersburg-headquartered Transpetrochart, for carrying jet fuel to the Russian military in Syria.

On Monday, reports emerged that Russia fired warning shots at the 3,270-dwt Sukru Okan (built 1989) after the ship allegedly failed to respond to a request to halt for an inspection.

Mykhailo Podolyak, senior advisor to Ukrainian President Volodymyr Zelenskyy said in a post on social media platform X, that the incident was a “clear violation of international law of the sea” as well as an act of piracy.