BEIRUT -- The leader of al-Qaeda called on rival Islamic groups in Syria to end their infighting and focus on battling President Bashar Assad's forces in a recording released Thursday.

The call by Ayman al-Zawahri came as activists said that fighting between the al Qaeda-linked Islamic State of Iraq and the Levant and an array of other Islamic militant groups intensified in northern Syria.

The Britain-based Syrian Observatory for Human Rights said "Islamic State" fighters fully captured the northern town of Manbij on Thursday after days of fighting.

The Observatory said that 1,395 people, mostly rebel fighters, have been killed since the infighting began Jan. 3, the worst clash among opposition groups since Syria's crisis began in March 2011.

Al-Zawahri said the internal fighting "between the holy warriors of Islam has bloodied our hearts" and that it should stop immediately. He called on Islamic groups in Syria to set up an Islamic court that would mediate and resolve their 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 audio message was posted online Thursday to websites commonly used by militants groups. Its authenticity could not be independently verified, though it corresponded to previous messages by al-Zawahri.

Meanwhile Thursday, six mortar shells struck central Damascus near the upscale Abu Rummaneh neighbourhood, 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 LCC. The Observatory had a higher death toll of 160. Activists say more than 130,000 people have been killed in Syria since the uprising began.