French officials have taken the Indian captain of a suspected Russian shadow fleet tanker into custody days after the oil tanker was seized.

On Thursday, the French navy intercepted the tanker – named the Grinch – which President Emmanuel Macron said was “subject to international sanctions and suspected of flying a false flag”.

The Grinch had been travelling through the Mediterranean Sea from the Russian Arctic port of Murmansk. It is now moored, under guard, at a southern French port near Marseille.

Although Moscow is yet to comment, Macron said on Thursday that the shadow fleet helped to “finance Russia’s war of aggression against Ukraine”.

The office of the Marseille prosecutor office said the rest of the ship’s crew members – all also Indians – were being “kept on board” while the 58-year-old captain was taken into custody.

“The investigation aims to verify the validity of the flag used by the tanker,” prosecutors said. French media report say it was sailing under a Comoros Islands flag.

Nautical and air exclusion zones have been established around the anchorage site, officials say.

Announcing the seizure on Thursday, Macron said: “We are determined to uphold international law and to ensure the effective enforcement of sanctions.”

Many Western countries imposed sanctions on Russian energy following Russia’s full-scale invasion of Ukraine in 2022.

Earlier in January, British armed forces supported a US operation to seize a Russian-flagged tanker in the Atlantic that US officials said had broken sanctions by carrying oil for Venezuela and Russia.

Last October, France seized another sanctioned tanker, the Boracay, off its west coast before releasing it a few days later.

Shadow fleets are becoming increasingly common, with Venezuela, Iran and Russia all accused of using them to avoid sanctions on oil.

Financial intelligence firm S&P Global estimates that one in five oil tankers worldwide are used to smuggle oil from sanctioned countries.