France intercepted a sanctioned Russian oil tanker in the Atlantic on Sunday with the support of allies including the UK, French President Emmanuel Macron has said.

He said the French Navy boarded the Tagor, which maritime authorities said had been flying a false flag when it was detained around 400 nautical miles west of Brittany in international waters.

"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 for more than four years," Macron wrote on X.

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 so-called "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 such ship that France has boarded since September 2025.

Macron said the operation had been undertaken "in strict compliance with the law of the sea".

"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.

But almost 200 Russian "shadow fleet" vessels had entered UK waters as of 11 May since the prime minister threatened to intercept them in mid-March, BBC Verify analysis suggests.

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