German daily newspaper Frankfurter Allgemeine Zeitung in an interview asked the French president if he was "in favour of shooting down a Russian fighter jet if it enters European airspace without authorisation".

"In accordance with the doctrine of strategic ambiguity, I can tell you that nothing is ruled out," Macron replied.

After Russia was blamed for drone incursions into NATO members Poland and Romania, Macron last week said the alliance's response would have to "go up a notch" in the case of "new provocations" from Moscow.

Romania's defence ministry said last month the country's airspace had been breached by a drone during a Russian attack on infrastructure in neighbouring Ukraine.

NATO member Poland also said last month it had shot down Russian drones that violated its airspace as Moscow launched a barrage against Ukraine.

Macron earlier on Wednesday said France was investigating an EU-sanctioned, Russian-linked oil tanker anchored off the French coast for what he said were "serious offences".

The Boracay, a Benin-flagged vessel, has been blacklisted by the European Union for being part of Russia's sanction-busting "shadow fleet".

According to the specialist website The Maritime Executive, the vessel is suspected of being involved in mystery drone flights that disrupted air traffic in Denmark in September.