Syrian troops clashed with rebels on the southern outskirts of Damascus on Tuesday, activists said, in a flare-up that is part of a weeks-long government push to advance and retake opposition-held areas.

The latest fighting centred around the suburb of Hejeira, one of a patchwork of sprawling neighbourhoods and towns south of the Syrian capital that have been opposition strongholds for the past year.

The British-based Syrian Observatory for Human Rights, which has a network of activist on the ground, and a spokeswoman for a Damascus-based Syrian rebel council said President Bashar Assad's troops were backed by Shia fighters from Iraq and the Lebanese militant group Hezbollah. The spokeswoman spoke on condition of anonymity, fearing for her safety.

Further blow to rebels

Emboldened Assad troops have in recent weeks made advances on the battlefield, taking back at least four strongholds in the northern province of Aleppo and south of Damascus. This week, government forces recaptured the town of Sbeineh, adjacent to Hejeira.

In a possible further blow to the rebels, a ceasefire deal was struck Tuesday in the Palestinian-dominated neighbourhood of Yarmouk in southern Damascus, reported the Syrian state media and also the Observatory's chief, Rami Abdurrahman.

Abdurrahman said the deal entailed that both rebel and pro-Assad gunmen lay down their arms. Government forces would be posted on checkpoints on the outskirts of Yarmouk, he said.

Other activists could not immediately confirm the deal, and there were still reports of fighting Tuesday in Yarmouk. Fighting in the neighbourhood, which is also home to a sprawling refugee camp, has displaced thousands of Palestinians.

Assassinated by motorcyclist

The deal, if implemented, will be one in a series negotiated in recent months between rebels, local community leaders and government officials. Such deals have allowed civilians to leave besieged areas, aid to enter and have even brought a halt to the fighting.

Meanwhile, in neighbouring Lebanon, masked gunmen riding a motorcycle assassinated a Sunni sheik in a volatile northern city, the official National News Agency reported. The city has been plagued by sectarian clashes linked to Syria's civil war, now in its third year.

Two attackers opened fire at Sheik Saad El-Deen Ghieh's car as he was driving by in Tripoli, 85 kilometres north of Beirut, the NNA said. Ghieh was taken to hospital and died shortly afterward.

Following the slaying, Lebanese troops were deployed to the area to prevent another outbreak of fighting there. Tripoli has seen cyclic outbreaks of sectarian clashes, with two impoverished neighbourhoods bitterly split on their Syrian loyalties between supporters and opponents of Assad's government.

Scores killed

The Bab Tabbaneh district is largely Sunni Muslim, as are Syria's rebels who have led the armed uprising against Assad's rule. The Jabal Mohsen neighbourhood is dominated by residents of Assad's Alawite sect, a Shia offshoot.

Past street battles have killed scores of Tripoli residents, and two car bombs outside Sunni mosques in August killed at least 50 people.

Several Tripoli residents, speaking on condition of anonymity because they did not want to be perceived as involved in the sectarian tensions, said the assassinated sheik belonged to a Sunni organization that has good relations with Hezbollah.

Another member of the organization, known as the Islamic Work Front, was arrested in August on suspicion of involvement in the Tripoli mosque bombings.