French troops have boarded the deck of a tanker alleged to be from Russia's 'shadow fleet' and suspected of involvement in drone flights over Denmark last month.

A source within the executive branch told AFP earlier that the French navy had boarded the Boracay, a Benin-flagged vessel blacklisted by the European Union for being part of Russia's sanction-busting fleet of ageing oil tankers.

French President Emmanuel Macron did not confirm reports of a connection to Danish drone flights but said on Wednesday that the ship had committed 'serious offences'.

It is currently at anchor near the western French city of Saint Nazaire.

The Boracay tanker was detained earlier this year for sailing without a valid country flag.