At least 482 people have been killed in clashes between Syrian rebels and the al-Qaeda-linked Islamist State in Iraq and the Levant (ISIS), activists say.
The Syrian Observatory for Human Rights said the toll comprised 85 civilians, 240 rebels and 157 ISIS fighters.
ISIS had killed 42 prisoners in Aleppo, while 47 members of the jihadist group had been executed by rebels, it added.
The fighting has spread across four provinces in rebel-held parts of northern Syria over the past week.
Attacks on fellow rebels and the abuse of civilian opponents of President Bashar al-Assad’s government by ISIS’s predominantly foreign fighters have led to increasingly frequent confrontations in recent months.
The latest clashes erupted last Friday when rebels led by the Islamic Front, a relatively new coalition of Islamist groups, launched what appeared to a series of co-ordinated strikes against ISIS. The offensive was backed by the opposition National Coalition.
Islamist brigades captured ISIS’s headquarters in the northern city of Aleppo on Wednesday. At the former children’s hospital they found the bodies of several men who had been executed.
On Friday, rebels were reportedly making advances against ISIS in Aleppo and Idlib provinces, where ISIS’s presence was relatively weak, but struggling in the ISIS stronghold of Raqqa, large parts of which have been under the jihadists’ control for months.
Rami Abdul Rahman, the director of the Syrian Observatory, told the AFP news agency: “It is likely dozens more people have lost their lives, but it is impossible to accurately document all the killings.”
On Tuesday, an ISIS spokesman warned its rivals that it would “crush them completely and kill the conspiracy in its cradle”.
The BBC’s Jim Muir in Beirut says this has been by far the worst bout of violence between rebel groups since the uprising began in March 2011 and it is yet to run its course.
Despite the severity of the conflict, there has been little sign yet that the overall rebel effort against the regime has been affected, our correspondent adds.
Fighting in that war continues, especially in the country’s third biggest city, Homs, where government forces killed about 40 rebels as they sought to break a siege, and in Damascus.
A UN official warned on Friday that the humanitarian situation in the besieged, rebel-held Yarmouk Palestinian refugee camp in the south of the capital was deteriorating.
“The profound civilian suffering in Yarmouk deepens,” Christopher Gunness told the Associated Press.
“Residents, including infants and children, have been subsisting for long periods on diets of such things as stale vegetables, animal feed and cooking spices dissolved in water,” he added.
“Infants are suffering from diseases linked to severe malnutrition, including anaemia, rickets, and kwashiorkor [a protein deficiency].”
The Syrian Observatory said separately that it had documented the deaths of 41 Palestinian refugees as a result of food and medical shortages in the past three months.
The violence comes less than two weeks before the planned start of an international conference in Switzerland to find a political solution to the conflict, which the UN says has left more than 100,000 people dead.
Russian media reported on Friday that Russian Foreign Minister Sergei Lavrov, US Secretary of State John Kerry and the UN and Arab League envoy for Syria, Lakhdar Brahimi, would hold talks in Paris on Monday to discuss issues surrounding the so-called Geneva II talks.
On Thursday, French Foreign Minister Laurent Fabius said he was not sure they would take place, while the Syrian Muslim Brotherhood said conditions were not right.
Questions remain over who will represent the Syrian opposition and whether Iran will play a role.
About 180 representatives of opposition factions who met in the Spanish city of Cordoba on Friday reportedly could not agree who should attend Geneva II, or whether they would attend at all. However, they agreed the talks should focus on establishing a time-frame for an end to the fighting.