Dozens of people are dead or missing after an air strike hit a school where displaced families had sought refuge near the Daesh-held city of Raqqa The activist-run Raqqa Is Being Slaughtered Silently group said about 50 families had been sheltering at the school in the northern Syrian village of Mansoura and that their fate was still unknown.

Mansoura is 16 miles west of Raqqa, the de facto capital of the extremists' self-described caliphate, and is under their control.

The British-based Syrian Observatory for Human Rights said 33 bodies had been pulled from the rubble.

The two organisations rely on local contacts to smuggle news out of Islamic State-held territory.

It was not immediately clear who carried out the air strike. Syrian Kurdish forces have been advancing on Raqqa under the cover of US-led coalition air strikes and are now five miles north of the city. Syrian and Russian aircraft have also carried out strikes against the Daesh group.

Elsewhere in Syria, insurgents advanced on government-held towns and positions north of the central city of Hama, the country's fourth largest city.

An al Qaida-linked group spearheaded the assault, launched on Tuesday, by detonating a car bomb in the nearby town of Souran.

The activist-run Hama Media Centre said the rebels had reached the village of Khatab, six miles north west of government-held Hama. State media denied reports of the advance.

The offensive coincides with a concentrated effort by opposition forces to breach government lines in the eastern neighbourhoods of the capital Damascus. That operation is also spearheaded by the al Qaida-linked Levant Liberation Committee.

In Damascus, the insurgents were pinned down in industrial zones in the city's east.

Government air strikes and artillery fire reverberated through the capital, and smoke could be seen rising above the opposition-held Jobar neighborhood.

Residents close to the front lines on the government side were seen leaving their buildings with whatever belongings they could pack in their suitcases.

AP The increase in fighting precedes the resumption of UN-mediated talks between the government and the opposition in Geneva due to get under way on Thursday.

UN Special Envoy to Syria Staffan De Mistura called the developments "alarming," but said the invited delegations would be in attendance "with, I hope, serious intentions to follow up the political process".

He spoke to reporters in Moscow following a meeting with Russian Foreign Minister Sergei Lavrov.

Mr Lavrov accused militants of carrying out more "terrorist attacks," which he said were "aimed at disrupting or complicating the Geneva talks".

Russia is an ally of Syrian President Bashar Assad.