French soldiers have boarded an oil tanker believed to be part of Russia's shadow fleet, used to evade sanctions imposed because of the war in Ukraine.
The Boracay left Russia last month and was off the coast of Denmark when unidentified drones forced the temporary closure of several airports last week. It has been anchored off western France for a few days.
President Emmanuel Macron said the crew had committed serious offences at an EU leaders' summit in Copenhagen on Wednesday but did not elaborate.
Kremlin spokesman Dmitry Peskov said Russia had no knowledge of the vessel.
AFP news agency quoted a source as saying French military personnel had boarded the vessel on Saturday.
Macron refused to be drawn on the question of whether the ship may have been used as a platform for the drone flights that caused such disruption in Denmark last week.
Prosecutors in Brest have opened an investigation on two counts: refusing an order to stop and failing to justify the nationality of the ship's flag.
Many Western countries imposed sanctions on Russian energy by limiting imports and capping the price of its oil following Russia's full-scale invasion of Ukraine in 2022.
To evade these sanctions, Moscow has built up what has been referred to as a shadow fleet of tankers whose ownership and movements can be obscured.
Russia is believed to have a fleet of several hundred tankers registered in other countries used to export its petrol, with Macron estimating that the shadow fleet comprises between 600 and 1,000 ships.
The Boracay, also known as Pushpa and Kiwala, is a Benin-flagged vessel but has been listed under UK and EU sanctions on Russia. It was detained by Estonian authorities earlier this year for sailing without a valid country flag.
Having left the Russian port of Primorsk on September 20, the Boracay sailed through the Baltic Sea and past Denmark before entering the North Sea and moving along the English Channel. It was expected to arrive in Vadinar, India on October 20, according to tracking data. However, after rounding Brittany, it altered course towards the French coast while being followed by a French warship.
EU leaders have been meeting in Copenhagen amid pressures to boost European defense in light of various Russian incursions into EU airspace, coinciding with drone disruption at airports in Denmark. Danish Prime Minister Mette Frederiksen emphasized the need for a united European response to perceived Russian threats.
While no evidence directly links the drone disruptions to Russia, the incidents add to a pattern of hybrid threats faced by countries, particularly those on the EU's eastern flank, prompting discussions of enhanced defense mechanisms like a multi-layered drone wall among member states.
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