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 stated the crew had committed serious offences at an EU leaders' summit in Copenhagen on Wednesday but did not elaborate.

Kremlin spokesman Dmitry Peskov responded that 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 whether the ship may have been connected to the drone flights that caused significant disruption in Denmark last week.

Prosecutors in Brest have initiated 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 circumvent these sanctions, Moscow developed a shadow fleet of tankers whose ownership and movements could be obscured.

Estimates suggest Russia operates a fleet of several hundred tankers registered in various countries to export its fuel. Macron indicated that 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 that has been subject to UK and EU sanctions regarding Russia. It was previously detained by Estonian authorities earlier this year for sailing without a valid country flag.

Having departed from the Russian port of Primorsk near Saint Petersburg on September 20, the Boracay traveled through the Baltic Sea, past Denmark, entered the North Sea, and continued through the English Channel. It was expected to reach Vadinar in north-western India on October 20, according to Marine Traffic tracking data. However, after rounding the Brittany coast, a French warship followed it, leading to a change in course towards the French coast.

EU leaders met in Copenhagen amidst calls to strengthen European defense capabilities after recent Russian incursions into EU airspace, which included drone disturbances targeting Danish airports. Danish Prime Minister Mette Frederiksen condemned what she described as a threat from Russia, highlighting the need for a robust response.

Despite Danish police finding no evidence linking Russia to the drone activity, Frederiksen connected these incidents to a broader pattern of hybrid attacks, indicating that European nations must unite against such threats.

The situation reflects the escalating tensions in Europe regarding security and defense in light of ongoing Russian military actions and potential threats.