At least 29 rebels have died in a blast in the central Syrian city of Homs as they primed a car bomb for an attack.

"The death toll is likely to rise because there are dozens of people missing and body parts in the area of the blast," the Syrian Observatory for Human Rights (SOHR) monitoring group said.

State news agency SANA also reported the blast, saying a car had exploded while being loaded with explosives.

One activist network, the Syrian Revolution General Commission, said the blast was the result of a rocket landing on an ammunition depot in the area. The claim could not be independently confirmed.

The blast took place on the outskirts of the Old City of Homs, which is under rebel control.

Some 1,400 civilians were able to leave the area this year under UN supervision, but an estimated 1,500 people remain.

In the capital, meanwhile, SANA said two people were killed in mortar fire by rebel fighters.

The attack damaged the Damascus Opera House, which was inaugurated by president Bashar al-Assad in 2004 near key government and military buildings on Umayyad Square.

The rebel fire on Damascus comes as government forces step up a campaign to crush insurgents in its eastern suburbs.

On Sunday, SOHR said five civilians, including three children, were killed in regime air strikes on the town of Douma, north-east of Damascus.

And additional air raids as well as fierce fighting was reported in Mleiha, south-west of the capital in Damascus province.

In northern Aleppo province, SOHR said two people, including a child, were killed in raids using explosive-packed barrel bombs, an army tactic that has caused dozens of deaths.

AFP