Syrian warplanes have bombed a Damascus suburb as heavy fighting rages for the second day on the outskirts, challenging President Bashar al-Assad's hold on the capital.

Fighter jets hit the suburb of Daraya near the main southern highway, where rebels have been battling elite Republican Guard units.

The pro-government al-Ekhbariya television said the army had begun a campaign to "cleanse" Daraya of what it described as terrorists.

The station showed troops on the edge of the town, where activists reported 23 people killed in two days.

But rebels and activists suggested that President Bashar al-Assad's forces were finding it harder to dislodge the rebels than when they last entered the suburb in August.

After months of slow progress, the rebels have in the last few weeks captured several army positions on the outskirts of Damascus and outlying regions, including a special forces base near Aleppo, Syria's commercial hub.

Rebels have also seized an air defence position near the southern gate of the capital.

Assad's opponents are also gaining some support internationally as a newly formed coalition of opposition and rebel groups seeks recognition as the legitimate voice of the Syrian people.

The British Royal United Services Institute Shashank Joshi said that the developments of the last few weeks were shifting the balance in favour of the rebels.

Fighting was also reported in Damascus's eastern suburb of Irbin, where rebels said they had destroyed one tank and killed two Republican Guards.

Irbin is one of many Sunni Muslim suburbs in the farmland around Damascus known as al-Ghouta.

Severe restrictions on non-state media makes it impossible to verify such reports independently.