Al Qaeda leader Ayman Al Zawahiri
Al Qaeda leader Ayman Al ZawahiriReuters

Al Qaeda leader Ayman al-Zawahiri has ordered the group's official branch in Syria, the Nusra Front, to halt fighting against its rivals in the Islamic State of Iraq and the Levant (ISIS), after months of bloody clashes have weakened both groups and undermined the rebel movement to overthrow the Assad regime.

In an audiotape posted online Friday, Zawahiri addresses Nusra Front head Abu Mohammed al-Jolani directly, and orders that "all soldiers of the front immediately cease fighting" rival Islamist groups. Instead, he urges Jolani to "devote himself to combat the enemies of Islam, specifically Baathists, Shiites and their allies" - a reference to the Baathist regime of Syrian President Bashar al-Assad.

More gains for Assad

Infighting between Syrian rebels has helped the Assad regime, backed by foreign Shia fighters from Lebanon, Iraq and Iran, to turn the tide of the three-year civil war in their favor, retaking swathes of territory and moving closer to gaining full control over Syria's major cities.

In the latest such development, a ceasefire was reached Friday in the country's third-largest city of Homs, where rebel fighters have been holed-up in the face of a prolonged regime bombardment and siege that has claimed countless lives, according to AP.

The ceasefire will allow several hundred remaining rebel fighters to evacuate the city, clearing the way for pro-government forces to retake control, in a move which would mark a significant strategic victory for Assad and a major blow to rebels.

The split between Al Nusra and ISIS - the latter of which is still Al Qaeda's official branch in Iraq - stems from a power-struggle between Jolani and ISIS head Abu Bakr al-Baghdadi. Zawahiri has thrown his weight behind Jolani, and in his most recent message called on al-Baghdadi to "devote himself to wounded Iraq, which needs you to redouble your efforts" there.

ISIS's involvement in the Syrian civil war "has been a political disaster for Syrians" and a "gift to Assad" by sowing division among Islamist rebels, he asserted.

Some rebel groups have gone further, accusing ISIS of actually working on behalf of the regime. An alliance of rebel groups, including the Nusra Front, have succeeded in driving ISIS from key areas in northern Syria. However, the group still retains a firm grip on Raqqa in northern Syria, where it has been accused of a wide range of war crimes - including crucifying its opponents - after imposing a strict version of Islamic law on the local population.