France's anti-terrorism prosecutor says about 40 convicted terrorists are due to be released from French prisons this year and next, calling the re-entry into society of the unrepentant ones a "major risk."

Francois Molins said on BFM television Monday that France still faces a significant threat from homegrown supporters of the struggling Islamic State group.

Molins estimated that 600 to 700 French extremists are unaccounted for in the areas of Iraq and Syria claimed by IS, though many have probably been killed. He says authorities think some organizers of the November 2015 attacks in Paris are among the dead in the Mideast.

Molins says the investigation of the simultaneous attacks at the Bataclan concert hall, Paris cafes and the national stadium that killed 130 people should conclude next year.