At least eight Syrian rebels were killed overnight when jihadists from the Islamic State of Iraq and the Levant detonated a car bomb in Syria's Idlib province, an NGO said.

The Syrian Observatory for Human Rights said the car bomb targeted a rebel checkpoint near Ram Hamdan, northeast of Idlib city.

"Eight fighters from Islamist and other rebel brigades died after a huge bombing carried out by ISIL fighters," the Observatory said.

Clashes between coalitions of Islamist and moderate rebels, who are fighting to topple President Bashar al-Assad's regime, and their erstwhile allies ISIL erupted nearly two weeks ago, after a spate of accusations of abuses by the jihadist group.

Among those battling ISIL is the Islamic Front, a coalition of Islamist battalions, which issued the group an ultimatum late Monday night.

"We are setting ISIL a deadline of no more than 24 hours to stop its abuses, free the prisoners and hand over its weapons," the statement said.

If the group failed to heed the warning, the Front said it would "deal with ISIL's presence and actions the same way as with the Assad regime."

Rebel groups have set ISIL similar deadlines in areas where the fighting in taking place, which the jihadist group has ignored.

The clashes were continuing on Tuesday, the Observatory said, including in Raqa, a stronghold of ISIL and the only regional capital to fall from regime hands.

The World Health Organisation and UN Children's Fund UNICEF said on Monday that the fighting there had interrupted a polio vaccination campaign launched after 17 cases of the disease were reported in Syria.

Elsewhere, the Observatory said 18 people had been killed in the Ghouta neighbourhood of Homs city in central Syria by shelling.

Fighting between the Syrian regime and rebels has continuing alongside the battles between opposition fighters and ISIL.

More than 130,000 people have been killed in Syria since conflict broke out in the country in March 2011.