The French Navy on Sunday boarded an oil tanker, named the Tagor, which was subject to international sanctions and sailing from Russia, French President Emmanuel Macron wrote on X.

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“This operation took place in the Atlantic Ocean, on the high seas, with the support of several partners, including the United Kingdom, in strict compliance with the law of the sea,” he said.

“It is unacceptable for ships to circumvent international sanctions, violate the law of the sea, and finance the war that Russia has been waging against Ukraine for more than four years,” he added.

The Maritime Prefecture of the Atlantic said in a separate statement on Monday that the French Navy had intervened on an oil tanker more than 400 nautical miles west of the tip of Brittany, coming from Murmansk, Russia.

“This operation was aimed at checking the nationality of a vessel suspected of flying a false flag. After the inspection team boarded the vessel, an examination of the documents confirmed suspicions regarding the irregularity of the flag flown. In accordance with international law and at the request of the public prosecutor, the vessel was diverted,” it added.

The prefecture did not name the ship.

There was no immediate reaction from Moscow.

In January, France, working with intelligence provided by the U.K., intercepted another oil tanker in the Mediterranean Sea that traveled from Russia. At the time, French maritime authorities for the Mediterranean also said the ship, the Grinch, was suspected of operating with a false flag.

France and Britain have both vowed to obstruct ships linked to Russia’s sanctioned “shadow fleet” that pass through their waters, and illegally carry sanctioned Russian oil or goods to be sold on the black market elsewhere, helping Russian President Vladimir Putin finance his war effort in Ukraine.

British Prime Minister Keir Starmer announced in March that he had granted permission for the U.K. military to board ships belonging to the “shadow fleet.”

However, shipping data shows that dozens of sanctioned ships linked to Russia continue to cross U.K. waters.