Fierce fighting between jihadists and rival rebel groups in Syria has killed at least 700 in more than a week while hundreds more are missing, a monitoring group said Sunday.
The Al-Qaeda-affiliated jihadist group, Islamic State of Iraq and the Levant (ISIL), linked the violence to a peace conference on Syria slated to take place in Switzerland on January 22.
The allegations came as Western powers on Sunday stepped up pressure on Syria's divided opposition to take part in the conference alongside representatives of President Bashar al-Assad's regime.
"From January 3 to 11, the fighting killed 697 people, among them 351 rebels, 246 members of the Islamic State of Iraq and the Levant and 100 civilians," said the Syrian Observatory for Human Rights.
Observatory chief Rami Abdel Rahman said there could be even "more than 1,000" dead, but his group has been unable to document all the killings given the ferocity of the clashes.
The Britain-based monitoring group, 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.
The Observatory also reported that dozens of those killed in recent days died in 16 suicide bombings staged by ISIL in Aleppo, Idlib, Homs and Raqa provinces.
"Sixteen suicide attackers have detonated themselves in the past week, most of them in car bomb attacks, some using explosive belts," Abdel Rahman 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 via the Internet on condition of anonymity.
An ISIL commander had warned rival opposition fighters this week of car bomb attacks if they pressed their offensive against the jihadists.
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ISIL to strike 'with iron fist'
And on Sunday the ISIL branch in Raqa alleged the fighting that erupted nine days ago was linked to the January 22 peace conference known as "Geneva 2".
ISIL charged that a group of rival rebels launched the offensive against the jihadists on several fronts "for the sole objective of destroying (us)... and replacing us... to satisfy apostate states who will meet at Geneva 2".
ISIL, which is seeking to set up an Islamic state in Syria, said its militants retaliated "in order to protect the jihad" and vowed to "strike with an iron fist those who think they can spill the blood of Muslims".
The Swiss talks seek to revive a long-stalled framework for peace involving a cessation of hostilities and the creation of a national transitional government that could involve figures from the current regime and the opposition.
But opposition leaders are wary of being drawn into a process they fear could result in Assad clinging on to power and have yet to give a commitment to attending.
Fighting between ISIL and other rebels raged Sunday in parts of Raqa after jihadists seized much of the city, which is the only provincial capital not in regime hands.
Assad's army meanwhile took advantage of the infighting to push for more control of Aleppo province.
Regime aircraft dropped explosives-packed barrels on the towns of Al-Bab on Sunday, killing at least eight people.
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.
More than 130,000 people have been killed in Syria and millions more displaced since March 2011.