"This morning, the French Navy boarded and searched an oil tanker from Russia, subject to international sanctions and suspected of flying a false flag," he said on X.

"The operation was carried out on the high seas in the Mediterranean, with the support of several of our allies," he added, saying the vessel had been "diverted".

"We will let nothing pass," said Macron.

"The activities of the shadow fleet help finance the war of aggression against Ukraine," he added, referring to conflict sparked by Russia's 2022 invasion.

Local maritime authorities said the navy seized an oil tanker called "Grinch" between Spain and Morocco.

"After the team boarded, an examination of documents confirmed the doubts as to the regularity of the flag," the Mediterranean Maritime Prefecture said.

The ship "is currently being escorted by the national navy to a point of anchorage for further verifications," it added.