PARIS (AFP) - French special forces have been deployed in Libya to fight the Islamic State in Iraq and Syria (ISIS) group, France's Le Monde reported on Wednesday - triggering an official probe into a possible leak of classified information.

Le Monde said special forces and members of the DGSE external security service were present in Libya for "clandestine operations" against ISIS militants, thought to number several thousand in the chaos-scarred country.

It also claimed that an air strike in November that killed the top ISIS leader in the country, Abu Nabil, was "initiated by Paris".

French officials said it had launched a probe into the leak.

"The investigation should establish if details covered by defence secrecy rules were revealed in this article," said a source close to Defence Minister Jean-Yves Le Drian.

"When secret operations are taking place, the goal is to ensure they are not revealed for the security of the men and the operations," said the source.

A violation of defence secrecy carries a penalty of up to three years in prison and a €45,000 (S$70,000) fine.

France and other Western countries have agreed that military action is needed to dislodge ISIS in Libya, which has exploited the chaos caused by the overthrow of dictator Moamer Kadhafi in 2011.

But it wants a national unity government in place to request the assistance before making a formal intervention.