A barrage of air strikes on Monday killed 53 civilians, including 16 children, in a string of villages held by the Islamic State group in central Syria, a monitor said.

The Syrian Observatory for Human Rights said the raids hit Oqayrabat and other nearby villages in the central province of Hama at dawn.

The Britain-based monitor had earlier reported 34 civilians killed and "dozens of people wounded".

It warned that "the toll may rise because some of them are in critical condition and because of the lack of medical capacities."

Observatory head Rami Abdel Rahman said he could not confirm if the strikes on the east of Hama province were carried out by Syrian or Russian warplanes.

He said there were cases of suffocation but could not confirm accusations of a chemical attack.

A panel set up by the United Nations, known as the Joint Investigative Mechanism, has determined during a year-long probe that Syrian government forces carried out three chlorine gas attacks on villages in 2014 and 2015.

The panel consisting of UN and experts from the Organisation for the Prohibition of Chemical Weapons (OPCW) also found that IS -- which seized a large swathe of Iraq and Syria in 2014 -- was behind a mustard gas attack in Syria in August 2015.

Oqayrabat lies northwest of Palmyra, the ancient desert city that was recaptured by IS at the weekend.

Syrian government forces, backed by their Russian ally, had driven IS from Palmyra in May.

The war raging in Syria erupted in March 2011 with anti-government protests demanding the ouster of President Bashar al-Assad.

But it has since morphed into a multi-front conflict that has drawn in world powers and witnessed the rise of jihadist groups.

More than 300,000 people have been killed and millions have been forced to flee their homes.