Rebel-on-rebel fighting has killed nearly 500 people over the past week in northern Syria, according to an activist group.

BEIRUT -- Days of fighting between rival rebel factions in Syria's contested northern and central regions have killed nearly 500 people, according to opposition activists.

The Britain-based Syrian Observatory for Human Rights, which operates a network of activists throughout the country, released the figures Friday, a week after clashes broke out between Al Qaeda-affiliated Islamic State of Iraq and Syria, or ISIS, and other rebel factions opposed to the government of President Bashar Assad. [Link in Arabic]

The infighting began in Aleppo province but quickly spread to the provinces of Idlib, Raqqah and Hama.

Of the 482 reported dead, 85 were civilians, many of them ISIS captives  executed as the group withdrew from its headquarters across northern parts of Syria, the Observatory said. ISIS had imprisoned journalists and activists opposed to its rule on charges such as not converting to Islam.

The group's many non-Syrian members have caused resentment among local populations with their extreme interpretation of Islamic law, or sharia.

"This is their Islam?" said a man in a video posted Wednesday on YouTube purporting to show the bloody corpses of ISIS prisoners.

Despite the group’s recent setbacks in Syria, more moderate opposition activists expect retaliation by ISIS members, using their signature tactics of car and suicide bombings. Pictures posted on the Facebook page of the self-styled ISIS emir, or commander, in the northern border town of Jarabulus showed cars laden with dozens of rusty canisters, presumably bombs.

"In Aleppo city, there is no one who says they are ISIS, but in the countryside there are still battles, and ISIS still controls some villages," said an opposition activist in Aleppo who goes by the name Ghaith Murjaan.

ISIS has also continued to fight and hold ground in the eastern rebel stronghold of Raqqah, the first Syrian city to fall completely into rebel hands. ISIS supporters released a video via social media showing a masked fighter celebrating the takeover of a building.

"We give you the news that we have overrun the headquarters of these dogs," declares a masked fighter, who is shown standing in front of a wall scrawled with the name of another rebel group, Ahrar al Sham, in green spray paint. "Raqqah is for us, and we will expand from here."

The fighting has further muddied the lines in the historically fractured Syrian armed opposition, turning those who once were brothers in arms or friendly rivals into outright enemies. Though many view the infighting as a way for more moderate fighters to distance themselves from the extremists and regain Western support, others point to the emergence of factions that appear little different from their former Al-Qaeda-linked companions.

The Islamic Front, one of the strongest opposition forces on the battlefield, includes some “very, very hard-line” Islamist groups, though they stop short of endorsing Al Qaeda’s call for global war against the West, said Aron Lund, who edits a website on the Syrian Crisis for the Carnegie Endowment for International Peace.

“The Front's declared ideology is one of a strict Islamic state, not an electoral democracy,” Lund said.

“But there are also factions in the Islamic Front that have joined it for pragmatic reasons. These are found especially on the lower levels, where whole units sometimes switch sides after organizational splits or deals between commanders and funders.... It's important to recognize those nuances and to realize that what comes out of a media office might not be representative of all of the thousands of fighters on the ground.”

The Islamic Front has repeatedly rejected calls for its participation in planned peace talks with Assad’s government. A statement releasedFriday  by one of the group's commanders denied a Reuters news report stating that its members had attended a meeting of opposition factions in Spain this week.

The report “is totally false, and the Islamic Front denies its participation in this meeting, for it declares that it will ... not accept any reconciliation solution with the regime," the statement said.

ALSO:

Twelve guides die in Mt. Everest avalanche

Britain's Labor Party hires ex-Obama aide Axelrod to help in election

South Korea ferry sinking: Arrest warrants sought for captain, 2 crew

Bulos is a special correspondent.