A Syrian activist group says airstrikes on the northern Syrian city of Raqqa and its outskirts over the past three days have killed at least 33 fighters with the Islamic State group.

The Britain-based Syrian Observatory for Human Rights says the fighters were mostly killed when their checkpoints were attacked by French and other warplanes.

France has launched waves of airstrikes against the IS group following attacks that killed at least 129 people in Paris on Friday. The Islamic State group has claimed responsibility for the Paris attacks.

The Observatory said on Wednesday that more IS fighters are believed to have been killed in the strikes, but their bodies were so severely dismembered it wasn't possible to give an estimated figure.