France has extended the detention of two senior crew members of a tanker linked to Russia, which data showed was off the Danish coast last month during mysterious drone flights, prosecutors said on Thursday.

The Boracay, a Benin-flagged vessel blacklisted by the European Union for being part of Russia's sanction-busting "shadow fleet" of ageing oil tankers, was stationed off Denmark from September 22 to 25, according to ship tracking data analysed by AFP.

Drones have been sighted across Denmark, including over military sites, since September 22, prompting brief closures at several airports and a ban on all civilian drone flights until Friday.

The tanker is now stationed of the coast of western France, and was stopped by French authorities on Saturday, according to a military source who

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