The French navy was Wednesday on board a tanker from Russia's "shadow fleet" stationed off France, AFP journalists at the scene said, after data showed it was off the Danish coast last month during mysterious drone flights. A government source, speaking on condition of anonymity, told AFP earlier the French navy had boarded the ship. But he stopped short of confirming reports of a connection to the Denmark drone flights. The French operation however underscored the importance of European efforts to stop the "shadow fleet" of vessels aiding Russia to circumvent Western sanctions. The French president said the "shadow fleet", estimated to include between "600 and 1,000 ships", was thought to represent "tens of billions of euros of Russia's budget" and make up "40 percent of the Russian war effort".