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.

According to news agency AFP, French military personnel boarded the vessel on Saturday. Macron refused to confirm if the ship was involved in the drone flights that disrupted operations in Denmark.

Prosecutors in Brest have started an investigation on two counts: refusing an order to stop and failing to justify the nationality of the ship's flag. Many Western countries have imposed sanctions on Russian energy, limiting imports and capping oil prices since the full-scale invasion of Ukraine in 2022.

To evade these sanctions, Moscow formed a shadow fleet of tankers whose ownership and movements are obscured. Russia is thought to have hundreds of tankers registered in other countries for exporting its petrol. Macron estimates that the shadow fleet consists of 600 to 1,000 ships.

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

Having departed from the Russian port of Primorsk, the tanker sailed through the Baltic Sea and past Denmark before heading towards the English Channel. It was tracked by a French warship as it changed course towards the French coast.

Meanwhile, EU leaders are under pressure to strengthen European defense following several Russian incursions into EU airspace. Danish authorities have linked recent drone disruptions to broader security threats, emphasizing the need for robust defensive measures.