On Thursday, the French navy intercepted the tanker — named 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 has yet to comment, Macron said the shadow fleet helps to “finance Russia’s war of aggression against Ukraine”.

The Marseille prosecutor’s office said the rest of the ship’s crew — all of whom are also Indian nationals — are being “kept on board”, while the 58-year-old captain has been taken into custody. “The investigation aims to verify the validity of the flag used by the tanker,” prosecutors said. French media reports say the vessel was sailing under a Comoros Islands flag. Nautical and air exclusion zones have been established around the anchorage site, officials added.

Announcing the seizure on Thursday, Macron said: “We are determined to uphold international law and ensure the effective enforcement of sanctions.” Many Western countries imposed sanctions on Russian energy following Russia’s full-scale invasion of Ukraine in 2022.

In January, British armed forces supported a US operation to seize a Russian-flagged tanker in the Atlantic that US officials said had violated 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 circumvent oil sanctions. Financial intelligence firm S&P Global estimates that one in five oil tankers worldwide is used to smuggle oil from sanctioned countries. (BBC)