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, the public prosecutor's office in the northwestern city of Brest said.

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

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, prosecutors said.

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

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

- European meeting -

Macron on Thursday urged Europe to 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 military chiefs of countries in the European-led "coalition of the willing" supporting Ukraine would hold talks "in the coming days" on how to disrupt the fleet.

Kremlin spokesman Dmitry Peskov said Russia did not have any information on the detained ship, and warned of "absolutely unpredictable consequences for the world's energy markets".

"President Macron says, let's hinder the movement of Russian oil. But what if you make a mistake and end up blocking all oil routes? And what if it's not Russian oil?" he told pro-Kremlin media outlet Izvestia.

Macron on Wednesday said 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, an armed forces source said.

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

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.

Some of these ships are suspected of also carrying out sabotage operations.

- 'No flag' -

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

Another 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 several other ships could have been used either as launch platforms or as decoys.

The tanker left the Russian port of Primorsk near 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-ah/as/sbk