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.

French President Emmanuel Macron said at an EU leaders' summit in Copenhagen on Wednesday that the crew had committed serious offences, but did not elaborate.

Kremlin spokesman Dmitry Peskov stated that Russia had no knowledge of the vessel.

News agency AFP reported that French military personnel had boarded the vessel on Saturday.

Macron declined to comment on whether the ship could have been used for the drone flights that disrupted Denmark.

Prosecutors in Brest have opened an investigation into the ship for violating orders and failing to justify its flag nationality.

Western nations have imposed sanctions on Russian energy, limiting imports and capping oil prices since Russia's invasion of Ukraine in 2022.

To evade these sanctions, Moscow created a shadow fleet of tankers with obscured ownership and movements, which is believed to include several hundred vessels registered under different countries.

Macron claimed Russia's shadow fleet could consist of between 600 and 1,000 ships.

The Boracay, also known as Pushpa and Kiwala, is a Benin-flagged vessel listed under UK and EU sanctions against Russia. Earlier this year, it was detained by Estonian authorities for sailing without a valid country flag.

After departing from the Russian port of Primorsk on September 20, the ship journeyed through the Baltic Sea, rounding Denmark before eventually heading towards the French coast under surveillance from a French warship.

EU leaders in Copenhagen are considering enhanced defense strategies due to a series of Russian actions, including recent drone disruptions over Danish airspace.

Danish Prime Minister Mette Frederiksen asserted that Russia poses a substantial threat to Europe, necessitating strong responses.

The ongoing incursions raise concerns for nations on the EU's eastern flank, prompting discussions about developing a multi-layered drone wall to counteract potential threats.