Syrian troops clashed with rebels Tuesday on the southern outskirts of Damascus, activists said, part of a weeks-long government advance to retake opposition-held areas around the capital and in the northern province of Aleppo.

Rebels seeking to overthrow President Bashar al-Assad have been weakened by infighting and what they say are waning weapon supplies from neighboring Turkey. A government policy of besieging rebel-held areas has also cowed fighters in some areas.

The fighting came as rebels intensified their mortar fire into the heart of Damascus — an often random shelling that has killed dozens of civilians this year.

On Tuesday, families in the Bab Sharqi neighborhood buried four Christian children who were killed a day earlier when mortar fire hit the bus that was taking them home from private school. The driver died as well.

The morgue visit was organized by Syrian officials who typically restrict reporters’ access to events.

Mortar shells also landed elsewhere in the capital Tuesday, including near the office of a pro-Assad Palestinian group, the state news agency said, adding that 10 people were wounded.

Tuesday’s fighting centered around the suburb of Hejeira, one of a patchwork of sprawling neighborhoods and towns just south of Damascus that have been opposition strongholds for the past year.

Government troops have taken control of four nearby strongholds in recent weeks, most recently the nearby town of Sabina.

Meanwhile, rebels in the northern city of Aleppo announced they were on high alert, fearing Assad-loyal forces would try to storm their eastern strongholds, according to the Britain-based Syrian Observatory for Human Rights and an Aleppo-based activist, Abu Raed.

Rebels ordered gunmen to present themselves for duty or be punished. “They want to halt the army’s advance,” Abu Raed said. “The regime is coming.”

Syrian troops have wrested back control of several areas around Aleppo city in recent weeks, including the town of Safira, a military base that oversees the Aleppo airport and the town of Tel Aran.

Meanwhile, in neighboring Lebanon, masked gunmen riding a motorcycle assassinated a Sunni sheikh in the volatile northern city of Tripoli, 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 moving car, the NNA said. Ghieh was taken to hospital and died shortly afterward.

— Associated Press