International efforts to crush the Islamic State group will increasingly have to extend to Libya where the militants are getting a hold, French Prime Minister Manuel Valls said Friday.

"We are at war, we have an enemy, Daesh, that we must fight and crush in Syria, in Iraq and soon in Libya too," Valls said, using an Arabic acronym for the group.

Speaking four weeks since ISIS gunmen and suicide bombers attacked Paris, Valls told France Inter radio the threat of further outrages remained "because we have hundreds, even thousands of young people who have succumbed to radicalisation".

French planes carried out surveillance flights over Libya last week.

Libya has slipped into chaos since the fall of Moamer Kadhafi in 2011 which ISIS has exploited. The UN believes 2,000 to 3,000 fighters are operating there, including 1,500 in the coastal city of Sirte.

International talks will take place in Rome on Sunday aimed at helping the country form a unified government.

France intensified its air strikes on ISIS's stronghold in Syria after the group claimed responsibility for attacking Paris.