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 on Monday.

 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 Monday 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, said “they use suicide attacks to terrorise society as a whole into submission, not just the fighters.”