This citizen journalism image provided by Aleppo Media Center AMC which has been authenticated based on its contents and other AP reporting, shows Syrians inspecting a burnt bus after a missile fired by Syrian government aircraft hit the vehicle in the rebel-held neighborhood of al-Bab in Aleppo, Syria, Tuesday, Dec. 31, 2013. The bus was full of people when it was struck, setting it on fire and killing several people, activists said. (AP Photo/Aleppo Media Center AMC)

BEIRUT - Syrian rebels seized a compound held by al-Qaida-linked militants Sunday as their one-time allies used car bombs against them, in some of the most serious infighting between fighters challenging the rule of President Bashar Assad.

The rebel-on-rebel fighting against the Islamic State of Iraq and the Levant is the strongest pushback yet from Syrian rebels who have seen their uprising to topple Assad hijacked by al-Qaida forces seeking to impose Islamic rule in opposition-held portions of the country.

The clashes began Friday after residents accused the al-Qaida-linked fighters of killing a doctor in the northern Syrian province of Aleppo. Fighting quickly spread to rebel-held areas of the northeast province of Idlib and the central province of Hama.

The clashes widened Sunday, with at least one outside of Aleppo pitting the ISIL against the Nusra Front, which is also an al-Qaida-aligned group, said Rami Abdurrahman of the Britain-based Syrian Observatory for Human Rights.

Another clash struck the town of Tabqa in the eastern province of Raqqa, where ISIL forces are most dominant, Abdurrahman said.

The Observatory obtains its information from a network of activists on the ground.

Rebels seized the ISIL compound in the town of Manbij in the northern province of Aleppo, activists said.

The Observatory said ISIL fighters also used car bombs for the first time to defend its territory. Other activists reported ISIL was pushed out of the town of Atmeh.

There always has been resentment against ISIL in Syria, whose fighters, a mix of foreigners and locals, fanned into the country last year, taking advantage of the upheaval to assert power in areas seized by rebels.

It is seen as particularly brutal in Syria's civil war for its abductions and killings of anti-Assad activists, journalists and civilians seen as critical to their rule.

But other residents welcome the group for chasing out thugs who terrorized residents in opposition-held areas, and for distributing food and aid to the poor.

ISIL emerged in the Sunni heartland of Iraq, where it has targeted Shiites with car bombs.

This week, they seized control of the key Sunni town of Fallujah, scattering government forces. It also claimed responsibility for a suicide bombing Thursday that targeted a Shiite-dominated Beirut neighbourhood.

Some activists hailed the fight against ISIL as a second revolution, but it wasn't clear if the fighting against the al-Qaida group could unite already chaotic rebel groups.

The groups battling ISIL come from a series of different brigade coalitions. Some are rivals to each other, and at least one group, the Army of the Holy Fighters, said they were battling the al-Qaida group because it did not properly accept Islamic law, in a video uploaded to the Internet.

The Western-backed coalition of Syrian opposition groups in exile has welcomed the attacks on ISIL, as it sees the group as hijacking its efforts to overthrow Assad.

But the potential loss of rebel-held territory as infighting persists also threatens to weaken Syria's already bruised opposition ahead of an international peace conference scheduled for this month to try hammer out the future of their country.

Syria's pro-government al-Watan newspaper appeared to welcome the infighting, offering this as a headline to an editorial Sunday: "Terrorism eats its sons."

News from © The Associated Press, 2014