Nearly 500 people, among them at least 85 civilians, have been killed in a week of fighting pitting Syrian rebels against jihadists in the north of the strife-torn country, a monitoring group said Friday.
The fighting raged as Western governments that back the revolt against President Bashar al-Assad prepared to intensify pressure on the opposition to participate in peace talks with the regime planned for later this month.
A new front opened last Friday in Syria's nearly three-year-old war, when powerful massive rebel groups combined to attack bases and checkpoints of the Islamic State of Iraq and the Levant (ISIL).
While the jihadists were initially welcomed by other rebels, allegations of brutal abuses against civilians as well as rival opposition fighters sparked a backlash, and even accusations that they were serving the interests of the regime.
"We have documented the killing of 482 people in the fighting -- 85 civilians, 240 members of the rebel brigades and 157 members of ISIL," said Syrian Observatory for Human Rights director Rami Abdel Rahman.
Among the civilians and rebels killed were 42 hostages who were executed in Aleppo by ISIL.
Rebels also executed 47 ISIL members, mainly in Idlib province in northwestern Syria, Abdel Rahman said, adding that dozens more were believed to have been killed in the fighting.
He called for "crimes being committed in Syria to be brought before an international court."
On Friday, rebels continued to advance in much of Aleppo and Idlib provinces, where ISIL's presence was relatively weak, while the jihadists had the upper hand in Raqa, which has been under their control for several months.
ISIL has its roots in Al-Qaeda in Iraq, and first appeared in the Syrian conflict in spring last year.
Civilians have suffered massively as a result of the latest fighting, activists say.
"In Aleppo city, people are trapped in their houses, unable to fetch medicine or food for fear they will get shot by snipers if they go outside," said anti-regime activist Alaaeddine.
"In Raqa, the situation is even worse," he added.
Despite the "numerical advantage" of Syria's rebels, "ISIL will not be forced out of Syria altogether," according to analyst Charles Lister, a visiting fellow at the Brookings Doha Centre.
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"It will maintain operations, but likely in a far more independent manner, and sometimes in opposition to other rebel groups."
ISIL has already carried out several deadly car bombings against rival rebel groups in recent days, particularly in Aleppo and Idlib.
Protesters meanwhile took to the streets Friday, as they have every week since the start of the revolt in March 2011, this time chanting slogans against both Assad and ISIL.
In the northern town of Binnish, protesters chanted: "Syria is free, free! ISIL, get out!" They also held up posters that read: "Bashar al-Assad is our main enemy."
41 dead in Yarmuk siege: NGO
Elsewhere in the country, the Observatory said at least 41 people had died of hunger or because of lack of medical care since September in the Palestinian Yarmuk camp south of Damascus.
The camp has been under a tight regime siege that has led to food and medicine shortages for some 20,000 people trapped inside.
The UN's agency for Palestinian refugees has warned about the situation in the camp, with UNRWA spokesman Chris Gunness describing "extreme human suffering."
The International Committee of the Red Cross said Friday that its chief was in Damascus to push for greater access for the group across the country.
"Our activities have expanded significantly over the last year, but we need to be allowed to do much more," said Peter Maurer, in a statement.
As the fighting continues, backers of the Syrian opposition have upped the pressure on dissidents to attend peace talks slated for January 22.
Speaking to AFP Thursday, veteran opponent and National Coalition member Samir Nashar said: "There are clear signs indicating the Coalition must go to Geneva."
The Coalition will meet on January 17 to decide whether to participate in the so-called Geneva 2 process, with large parts of the group opposed to attended.
The so-called Friends of Syria grouping of nations that back the opposition is expected to push for opposition participating at a Sunday meeting in Paris.
Syria's civil war has killed more than 130,000 people, and forced millions more to flee their homes.