PARIS (AP) -- French President Emmanuel Macron said Wednesday that an oil tanker off the French coast had committed "very serious wrongdoings" and linked it to Russia's shadow fleet, which is avoiding Western sanctions over Moscow's war in Ukraine.

The tanker was sailing last week off the coast of Denmark and was cited by European naval experts as possibly being involved in drone flights over the country.

Brest prosecutor's office said a judicial investigation has been open into the crew's "refusal to cooperate" and "failure to justify the nationality of the vessel."

The ship's current status wasn't clear. It left the Russian oil terminal in Primorsk near Saint Petersburg on Sept. 20, sailed off Denmark and has stayed off the coast of the French western port of Saint-Nazaire since Sunday, according to Marine Traffic monitoring website.

Macron suggested it was stopped by French authorities' "intervention," saying: "I think it's a good thing that this work has been done and that we've been able to stop it."

"There were some very serious wrongdoings made by this crew, which is why there are legal proceedings in the case," Macron said on the sidelines of a summit of European Union leaders in Copenhagen, Denmark. He didn't elaborate and France's maritime authorities did not immediately respond to a request for details.

Macron said the incident highlights "the existence and the reality of a phenomenon that we have been describing and denouncing for a long time" that is "these notorious shadow fleet" that represents tens of billions of euros for Russia's budget and finances an estimated 40% of Russia's war effort.

Macron said between 600 and 1,000 ships are transporting Russian oil and gas despite Western sanctions.

The tanker known as "Pushpa" or "Boracay," whose name has changed several times, was sailing under the flag of Benin and appears on a list of ships targeted by EU sanctions against Russia.