Rebels fighting the regime of President Bashar al-Assad on Saturday seized a key air base in the southern Syrian province of Daraa after two weeks of fierce battles with loyalist troops, a watchdog said.

In the ancient Umayyad mosque of Damascus, thousands of Assad supporters attended the funeral of pro-regime Sunni cleric Mohammad Said al-Bouti and his grandson, who were killed on Thursday in a suicide attack that left some 50 people dead.

"Opposition fighters loyal to Al-Nusra Front, Al-Yarmuk Brigade and other rebel groups seized air defence Base 38 near the town of Saida, on the road linking Damascus to Amman, in the province of Daraa," said the Syrian Observatory for Human Rights.

The seizure came "after 16 days of fighting," said the Britain-based group.

At least seven rebels were killed in their final assault on the base, said the Observatory, which also documented the deaths of at least eight regime troops including an officer.

"Dozens of prisoners were freed from the base's headquarters," it said.

Amateur video filmed by rebels and distributed by the Observatory showed the bloodied, mutilated corpse of a man identified as Mahmud Darwish, an officer.

Activists also distributed footage showing a group of men, most of them bearded, being freed.

The Syrian Revolution General Commission, a network of activists on the ground, said the rebels also captured a checkpoint in the Daraa town of Sahem al-Golan.

Amateur video showed rebels seizing at least two regime tanks and several military vehicles after they captured the checkpoint.

"I swear to God, we are coming for you, O Bashar" al-Assad, a rebel fighter said in a video distributed by anti-regime activists.

In Quneitra meanwhile, at least 35 rebels were killed on Wednesday and Thursday fighting troops loyal to Assad, said the Observatory, which reported at least 128 people killed across Syria on Friday.

Some 20 other fighters were also believed dead after battles in majority Druze villages in Quneitra province, which lays on the sensitive ceasefire line with Israel.

At Buti's funeral in Damascus, the country's top Sunni authority Mufti Ahmad Badreddine Hassoun called on "the Islamic and Arab world to save Syria, which is facing a global war".

"If Syria falls today you will be next," he said before thousands of mourners.

The ceremony was led by Toufik Bouti, the dead sheikh's son, while representatives of key Damascus allies Iran and Hezbollah also took part.

The United Nations estimates that violence across Syria has killed at least 70,000 people since the conflict erupted in March 2011.