The French navy on Saturday stopped the Boracay, a vessel claiming to be flagged in Benin and blacklisted by the European Union for being part of Russia's sanction-busting "shadow fleet" of ageing oil tankers, according to the public prosecutor's office in the northwestern city of Brest.

The vessel was investigated over inconsistencies in where the tanker was officially registered while it was carrying a "large cargo of oil" from Russia to India, it added.

The captain and first mate of the Boracay were detained on Tuesday over refusing to provide evidence of nationality or cooperate with French authorities, the prosecutor's office said.

The captain has been told to appear in court in February, but the first mate has been released, it said.

It was not immediately clear if the captain would be released under certain conditions or if the Russia-linked ship could go on its way.

Shipping data analysed by AFP has shown it was positioned off Denmark during mysterious drone flights over the country last month, including over military sites, that prompted brief closures at several airports.

French President Emmanuel Macron on Thursday urged Europe to "increase the pressure" on Russia's shadow fleet and follow his country's lead in detaining vessels used to fuel Moscow's war in Ukraine.

"You kill the business model by detaining, even for days or weeks, these vessels and forcing them to organise themselves differently," Macron told a leaders' gathering in Denmark.

He said on Wednesday France was probing the ship for "serious offences", but stopped short of confirming reports of a connection to the Denmark drone flights.

French military personnel remained on board the tanker on Thursday, another military source who spoke on the condition of anonymity said.

- 'Navy commandos' -

French Prime Minister Sebastien Lecornu on X thanked "navy commandos and the crews of the French navy who intervened this weekend aboard the tanker belonging to the Russian ghost fleet".

Estimated to include up to 1,000 ships, the shadow fleet is thought to represent "tens of billions of euros of Russia's budget" and make up "40 percent of the Russian war effort", according to Macron.

The vessels -- which fly flags of convenience, have opaque ownership, and often turn their transponders off -- enable Moscow to keep exporting its crude oil for much-needed revenue despite curbs on exports.

"The investigation carried out by the French Navy concluded that it had no flag," said the Brest public prosecutor.

The first military source told AFP that Russian ships were common off the French coast.

"Every day between 10 and 15 vessels from the shadow fleet sail off the coast of Brest," the source said.

The Boracay -- which has also been named the Pushpa or the Kiwala -- was positioned off Denmark from September 22 to 25, according to data from the maritime website VesselFinder analysed by AFP.

According to the specialist website The Maritime Executive, the 244-metre (801-foot) vessel is suspected of being involved in the drone flights over Denmark.

The publication said the tanker and other ships could have been used either as launch platforms or as decoys.

The tanker left the Russian port of Primorsk outside Saint Petersburg on September 20, shipping data showed.

Data from the Marine Traffic tracker showed the tanker was scheduled to arrive in Vadinar in northwestern India on October 20.

burs-ekf/ah/