At least 33 ISIS militants in Raqqa have been killed in air-strikes since Sunday as ISIS families flee the stronghold, local activists have said.

The Syrian Observatory for Human Rights (SOHR) said that many families of ISIS leaders had begun leaving the jihadists' main base for their other stronghold, the Iraqi city of Mosul, after air strikes have intensified in the wake of the Paris bombings.

Air strikes were carried out by Russian and French jets in the aftermath of the attacks on Paris on Friday night, said the UK-based group.

Local activists told SOHR that most of the casualties were at checkpoints around the city but an ISIS-linked news agency has reported no casualties from the air strikes. Amaq agency said that French warplanes had only hit "empty locations".

The journey between the two key ISIS-held cities, Raqqa in north Syria and Mosul in north-eastern Iraq, has become significantly more difficult as the main route between them was cut off last week. Kurdish forces recaptured the town of Sinjar in north-western Iraq, removing the vital link in ISIS' territory.

However, though ISIS leaders' families are leaving, civilians of Raqqa have been prohibited from fleeing, leading to fears they would be used as human shields in the event of a ground assault by US-backed Kurdish forces.

Despite the increased bombardment on ISIS' positions and French President Hollande's call for a "global coalition" against the jihadists, Russian foreign minister Sergei Lavrov warned yesterday that if western countries wanted a "genuinely global coalition," they would have to drop their demand for Syrian President Bashar al-Assad to step down.

""In my opinion, there can now be no doubts that it is simply unacceptable to put forward any preconditions in order to unite in the battle against so-called Islamic State terrorists," he said.