France said on Monday that a suspected Russian oil tanker had been detained in the Atlantic over the weekend, in the latest such seizure aimed at combatting Moscow's sanctions-busting 'shadow fleet.'

The Tagor was detained on Sunday morning in international waters with the help of Britain and other partners, said President Emmanuel Macron.

According to French authorities, the tanker was on its way from Murmansk in northwestern Russia when it was seized.

The ship was falsely flying a Cameroonian flag and was heading toward Limbe, a seaside city in the west of the African country, said a spokesperson for the maritime prefecture.

'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 4 years,' Macron said.

'These ships, which fail to comply with the most basic rules of maritime navigation, also pose a threat to the environment and to everyone's safety,' he added, posting a video that he said was of the seizure, which showed commandos rappelling from a helicopter onto a ship.

He added: 'This operation was carried out in the Atlantic, in international waters, with the support of several partners including the United Kingdom.'

An MoD spokesperson told the Daily Mail that a 'helicopter from onboard HMS Somerset provided tracking and monitoring in support of the French operation to board the tanker Tagor.'

They added: '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.'

The Atlantic maritime prefecture said the interception had taken place more than 400 nautical miles west of Brittany. 

'The examination of the documents confirmed doubts about the irregularity of the flag being flown,' the prefecture said.

The ship, which had 23 crew members, was 'being escorted by the French navy to an anchorage point for further checks,' the maritime prefecture said.

Guillaume Le Rasle, spokesman for the Atlantic maritime prefecture, said the tanker was under EU and US sanctions.

'It is a vessel that was known and tracked,' he said. 

'The decision to divert it was taken Sunday evening,' he added. 'The objective of the diversion is to verify the validity of its flag.'

The tanker, which has frequently changed flags, was 'almost empty' at the time of boarding, he added.

The last time it transmitted an automatic identification system (AIS) signal, a week ago, the Tagor was sailing off the Norwegian coast and flying a Madagascan flag, according to MarineTraffic tracker.

The 'shadow fleet' vessels frequently change the flags they fly, a practice known as flag-hopping, or use invalid registrations in an attempt to escape tracking.

Since September, France has boarded three other ships believed to belong to Russia's 'shadow fleet'. The ships were allowed to sail after their owners paid fines.

In September, the French navy boarded the Boracay, which claimed to be flagged in Benin. Its Chinese captain was put on trial in absentia, and a French court in March issued an arrest warrant and a one-year jail sentence against him.

In January, French forces impounded another suspected Russian tanker, the Grinch, and in March, the Deyna that sailed from Murmansk under a Mozambican flag was detained in Marseille.

In April, France announced a plan to double penalties for ships that fail to fly a flag or refuse to comply.

Several Western countries have imposed sanctions on hundreds of vessels in Russia's 'shadow fleet' over its 2022 invasion of Ukraine.

Nearly 600 vessels suspected of being part of Russia's 'shadow fleet' are subject to European Union sanctions.

Russian leader Vladimir Putin has condemned the detention of Russia-linked vessels as 'piracy.'