BEIRUT — The head of an al-Qaida affiliate in Syria called in an audio message on Sunday for other rebel groups to stop attacking his fighters and instead join forces to defeat President Bashar al-Assad.

The small but powerful al-Qaida arm,  the Islamic State in Iraq and the Levant (ISIL), has been caught up in clashes with other Islamist insurgents, often triggered by disputes over authority and territory. More secular rebels have also accused it of diverting from the original ideals of the revolt.

The internecine fighting - among the bloodiest seen in the whole three-year conflict - has undermined the uprising against Assad and dismayed Western powers pushing for peace talks. It has killed hundreds of rebels this month.

ISIL head Abu Bakr al-Baghdadi told rebels in an audio message posted on jihadi websites to "repent to God for you have stabbed us in the back while our soldiers were at the front."

He added that his group was "extending its hand to you, so you can stop fighting it and we will stop fighting you and we can fight the Alawites," referring to Assad's sect, an offshoot of Shi'ite Islam.

ISIL and the majority of rebels in Syria are Sunni Muslims.

The rebel groups fighting ISIL include Jabhat al-Nusra, al-Qaida's official Syria wing. In April, Baghdadi tried to merge ISIL with Jabhat al-Nusra, defying orders from al-Qaida chief Ayman Zawahri and causing a rift.

The al-Qaida wing is smaller than other rebel groups but its battle-hardened militants, many of the foreigners with experience fighting in other war zones, have regained lost ground in recent days.

Baghdadi also said that ISIL and the United States would soon be "in a direct confrontation and the sons of Islam have prepared for such a day."

Some 130,000 people have been killed and a quarter of Syrians driven from their homes in the civil war, which began with peaceful protests against 40 years of Assad family rule and has descended into a sectarian conflict, with the opposing sides armed and funded by Sunni Arab states and Shi'ite Iran.