New air strikes targeting the Syrian city of Raqqa have killed at least eight people, including five children, according to opposition groups.

The strikes came as French foreign minister Laurent Fabius declared that destroying the Islamic State headquarters and "neutralising and eradicating" the extremist group is the main objective of the international military campaign.

Raqqa, in north-eastern Syria, is the group's de-facto capital and has become the focus of air strikes in the wake of the Paris terror attacks and the bombing of a Russian jetliner over Egypt's Sinai Peninsula. IS has claimed responsibility for both incidents.

A Raqqa-based activist group that reports on IS, known as Raqqa Is Being Slaughtered Silently, said most of Friday's casualties occurred when warplanes targeted the city's Heten School.

The school, like others in Raqqa, has been taken over by IS.

The Syrian Observatory for Human Rights put the death toll at 12, including the five children.

It was not immediately clear who carried out the latest air strikes.