Fierce fighting between jihadists and rival rebel groups in Syria has killed at least 700 in more than a week while hundreds more have gone missing, a monitoring group said Sunday.

Among those killed were dozens of casualties in 16 suicide attacks staged by the jihadist Islamic State of Iraq and the Levant (ISIL), which is battling rebels mainly in northern Syria, said the Syrian Observatory for Human Rights.

"From January 3 to 11, the fighting killed 697 people, among them 351 rebel fighters, 246 members of the Islamic State of Iraq and the Levant and 100 civilians," it said.

Observatory chief Rami Abdel Rahman said the actual number of deaths could be at "more than 1,000", but his group has been unable to document all the killings given the ferocious nature of the combat.

The Britain-based Observatory, which relies on a network of activists across Syria for its reports, also said there were "hundreds of captives from both sides whose fate is unknown."

In a reflection of the brutality of the fighting, which has raged mainly in the northern provinces of Aleppo, Idlib and Raqa but has also hit Hama and Homs in the centre, at least 200 people were killed in one 48-hour period.

Dozens of the killings have been in a wave of suicide attacks by ISIL fighters targeting rival rebel positions.

An ISIL commander had warned rival opposition fighters earlier this week of car bomb attacks if they pressed their offensive against the jihadists.

"Sixteen suicide attackers have detonated themselves in the past week, most of them in car bomb attacks, some using explosive belts," Abdel Rahman said.

"Dozens of people in Aleppo, Idlib, Homs and Raqa provinces have been killed in such attacks," he told AFP.

On Saturday alone, 39 rebels were killed in the attacks in Aleppo, Idlib and Raqa provinces, the Observatory said.

A rebel fighter with Ahrar al-Sham, which is leading battles against ISIL in several areas, told AFP "they use suicide attacks to terrorise society as a whole into submission, not just the fighters."

"It is one of their most deadly weapons... which they use partly for a lack of other means," he said in a Skype call on condition of anonymity.

Fighting between the two sides raged in parts of Raqa on Sunday after ISIL managed to seize much of the city, which is the only provincial capital to have fallen out of President Bashar al-Assad's control.

The Observatory cited medical sources as saying there were at least 62 bodies of rebels in Raqa city hospital.

In Aleppo and Idlib, where most car bomb attacks have taken place, the jihadists were on the defencive on Sunday.

Hundreds of ISIL fighters were holed up in their base in Saraqeb in Idlib, a day after rebels captured most of the town.

Assad's army is, meanwhile, trying to take advantage of the infighting to push for more control of Aleppo province.

On Sunday government aircraft dropped explosive-packed barrels on the towns of Al-Bab, killing at least eight people, and Hreitan.

On Saturday loyalist troops seized the rebel town of Naqarin north of Aleppo and pushed toward the industrial part of Syria's second city, activists reported.

But rebels have launched a counter-offensive to recapture Naqarin, the Observatory said.

Meanwhile in the battered central city of Homs, the toll from the army's shelling of Waar district on Saturday rose to 21, the monitoring group said.

More than 130,000 people have been killed in Syria and millions more displaced since March 2011.

On Saturday alone, 232 people were killed nationwide, most of them rebels, jihadists and regime troops.

In Damascus, two more people died of malnutrition in the besieged Yarmuk Palestinian refugee camp, raising the toll of those who succumbed to hunger and medical shortage since September to 43.