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.