The Boracay, a Benin-flagged vessel blacklisted by the European Union for being part of Russia's sanction-busting "shadow fleet" of ageing oil tankers, was stationed off Denmark from September 22 to 25, according to ship tracking data analysed by AFP.

Drones have been sighted across Denmark, including over military sites, since September 22, prompting brief closures at several airports and a ban on all civilian drone flights until Friday.

French military personnel were Wednesday on the deck of the tanker, now stationed off the coast of western France, AFP journalists who overflew the area said.

Later Wednesday, Brest prosecutor Stephane Kellenberger told AFP that two crew members -- who presented themselves as the ship's captain and his first mate -- had been taken into custody.

A military source, asking not to be named, told AFP the vessel had been boarded on Saturday, with a government source confirming the boarding.

- France probes Russian ship -

President Emmanuel Macron earlier on Wednesday said France was probing the ship for "serious offences".

But he stopped short of confirming reports of a connection to the Denmark drone flights.

"There were some very serious offences committed by this crew, which justify the current judicial procedure," Macron told reporters at an EU leaders' summit in Copenhagen.

Built in 2007 and variously known as Pushpa and Kiwala, the Boracay has been anchored off Saint-Nazaire in western France for several days.

According to the specialist website The Maritime Executive, the 244-metre (801-foot) vessel is suspected of being involved in mystery drone flights that disrupted air traffic in Denmark in September.

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

But when asked about those claims, Macron said he would "remain very careful", as it was not for him to establish a link between the Boracay and the drone flights.

The French operation however underscored the importance of European efforts to stop the "shadow fleet" of vessels aiding Russia to circumvent Western sanctions.

- 'Refusal to cooperate' -

The European Union has sanctioned hundreds of ageing tankers used by Russia to circumvent oil export curbs imposed after the 2022 invasion of Ukraine.

Among them is the Boracay, which was blacklisted in February under the name Kiwala.

The public prosecutor's office in the northwestern French city of Brest said it had opened an investigation following a report from the navy.

The probe was launched over the crew's "failure to justify the nationality of the vessel" and "refusal to cooperate", Kellenberger, the prosecutor, told AFP.

The tanker left the Russian port of Primorsk outside Saint Petersburg on September 20 and was scheduled to arrive in Vadinar in northwestern India on October 20, according to data from the Marine Traffic tracking website.

The French president said the "shadow fleet", estimated to include between "600 and 1,000 ships", was thought to represent "tens of billions of euros of Russia's budget" and make up "40 percent of the Russian war effort".

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