BEIRUT (AP) -- The head of al-Qaida called on rival Islamic groups in Syria to end nearly three weeks of deadly infighting, telling them to set aside a bitter dispute that "has bloodied our hearts" and instead focus on battling President Bashar Assad's forces.
The appeal from Ayman al-Zawahri, which was posted online Thursday, came as activists said clashes showed no signs of letting up among the al-Qaida-linked Islamic State of Iraq and the Levant and an array of ultraconservative and more moderate rebel groups.
The violence, which broke out nearly three weeks ago and has spread across much of opposition-held northern Syria, is the worst bout of infighting among the Assad's opponents since Syria's conflict began in March 2011. It has thrown a spotlight on the fractured rebel movement, which even three years into the uprising has failed to create a unified armed opposition force -- let alone a command structure.
Assad's army, by contrast, has remained largely cohesive despite defections early in the revolt, and has bolstered its overstretched forces with pro-government militias and fighters from allies like the Lebanese Shiite militant Hezbollah group. Government troops have captured several villages southwest of Aleppo since the rebel infighting began.
Rival rebels initially pushed the Islamic State out of several towns, but the al-Qaida linked group has slowly clawed back some of its lost ground. On Thursday, its fighters seized full control of the town of Manbij, some 90 kilometers (55 miles) west of Aleppo, following some 10 days of fighting, the Britain-based Syrian Observatory for Human Rights said.
The Observatory said the death toll in 20 days of clashes has reached 1,395, most of them rebel fighters.
In his new audio message, al-Zawahri called for an immediate halt to the internal clashes, and called on Islamic groups in Syria to set up an Islamic court that would mediate and resolve the factions' differences. He said Islamic fighters should focus on "bringing down Assad's secular, sectarian, unjust and criminal regime to set up a just Islamic state."
Al-Zawahri's five-minute message was posted online to websites commonly used by militants groups. Its authenticity could not be independently verified, though it corresponded to previous messages by al-Zawahri.
The terror network leader's last significant message about Syria came in June, when he called on two powerful al-Qaida-linked groups in Syria -- the Islamic State and the Nusra Front -- to end their squabbling.
The Nusra Front has largely stayed on the sidelines of the infighting, which has mostly pitted the Islamic State against a powerful alliance of ultraconservative rebel groups known as the Islamic Front. Still, tensions between the two al-Qaida affiliates remain, and the Nusra Front's leader has criticized the Islamic State for its aggressive, even hostile, attitude toward many other opposition factions.
Meanwhile Thursday, six mortar shells struck central Damascus near the upscale Abu Rummaneh neighborhood, wounding a number of people, an Associated Press reporter in the area said.
It is not uncommon for rebels to fire mortar shells on Damascus, Syria's capital and an Assad stronghold. Dozens of people have been killed or wounded in the past month by such shelling.
The Observatory and the Local Coordination Committees, another activist group, reported violence in other parts of Syria on Thursday, including the central province of Homs, Daraa in the south and the suburbs of Damascus.
Thursday's fighting came a day after a peace conference aiming to end Syria's civil war began in Switzerland. The conference is slated to be the first direct talks between Assad's government and rebels.
At least 73 people were killed Wednesday, the first day of the conference, according to the LCC. The Observatory put the death toll at 160. Activists say more than 130,000 people have been killed in Syria since the uprising began.
Associated Press writers Albert Aji in Damascus, Syria, and Ryan Lucas in Beirut contributed to this report.
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