France intercepted a sanctioned oil tanker suspected of being part of Russia's so-called shadow fleet on Sunday with UK support, French President Emmanuel Macron has said.

Officers boarded the Tagor in the Atlantic, which maritime authorities said had been flying a false flag when it was detained around 400 nautical miles west of Brittany.

"It is unacceptable for ships to circumvent international sanctions, violate the law of the sea, and fund the war that Russia has been waging against Ukraine," Macron wrote on X.

A British helicopter provided support during the operation, the UK Ministry of Defence (MoD) told the BBC.

The Kremlin said the "illegal" seizure was "bordering on international piracy".

"Russia is taking measures to ensure the safety of its cargo," said spokesman Dmitry Peskov.

Moscow has been operating a "shadow fleet" of tankers with obscure ownership structures to evade international sanctions imposed on its oil exports since launching its full-scale invasion of Ukraine in 2022.

It is the fourth suspected shadow fleet vessel that France has boarded since September 2025.

Macron said the operation, which took place in international waters, had been undertaken with allies including the UK "in strict compliance with the law of the sea".

An MoD spokesperson said a helicopter which had been on board HMS Somerset provided "tracking and monitoring in support of the French operation".

"Alongside our allies, we are stepping up our response to shadow vessels – to choke off the funds that fuel Putin's illegal invasion of Ukraine."

"These vessels, which fail to adhere to the most basic rules of maritime navigation, also pose a threat to the environment and to everyone's safety," he said.

In a video shared by the French president, armed naval officers could be seen boarding the tanker via a helicopter. The BBC has not verified the footage.

Previously, French authorities had allowed these ships to continue operating after their owners paid fines but have since vowed to block them.

UK Prime Minister Sir Keir Starmer announced in March he similarly had granted permission for the UK military to board sanctioned Russian ships.

The MoD said at the time it was "disrupting and deterring" shadow fleet vessels, without providing specific details.