By Khaled Yacoub Oweis

AMMAN (Reuters) - Syrian warplanes bombed a Damascus suburb on Wednesday, opposition activists said, as heavy fighting raged for the second day on the outskirts, challenging President Bashar al-Assad's hold on the capital.

MiG fighter jets hit the suburb of Daraya, situated amid farmland near the main southern highway, where rebels have been battling elite Republican Guard units that have deployed around the town, a major opposition centre of the 20-month revolt.

The pro-government al-Ekhbariya television said the army had begun a campaign to "cleanse" Daraya from what it described as terrorists and showed troops on the edge the town, where activists reported 23 people killed in the last two days.

Assad's opponents are gaining some support internationally as a newly formed coalition of opposition and rebel groups seeks recognition as the legitimate voice of the Syrian people, with Britain becoming the ninth country to grant it such status.

A major offensive to oust Free Syrian Army fighters from Daraya in August killed 1,000 people after rebels took over the town, established a local administration and began attacking loyalist targets in Damascus, according to opposition sources.

"The military picture seems to have changed since August. The regime is sending troops under tank and air cover but they have not really advanced into Daraya," said Abu Kinan, an opposition activist who is still in the town, said by phone.

"Last time the rebels made a decision to withdraw after the army's bombing killed a large number of civilians. There are civilians left in Daraya but the bulk had fled and the fighters are holding their ground," he said.

Seven civilians and three rebels were killed in fighting and bombardments on Daraya, opposition sources said.

A couple died from shrapnel when artillery hit the basement of a building in which they were sheltering, activists said, and a video posted on YouTube showed the body of a baby at a hospital.

It was impossible to verify the reports independently. Syrian authorities have placed severe restrictions on non-state media.

After months of slow progress, the rebels have captured several army positions on the outskirts of Damascus and outlying regions in the last few weeks, including a special forces base near Aleppo, Syria's commercial hub, and an air defense position near the southern gate of the capital, according to activists, video footage and diplomats following the military situation.

Assad's core military units, composed mainly of members of his Alawite minority sect, an offshoot of Shi'ite Islam, are entrenched in Damascus, preventing any rebel push into the capital.

(Editing by Robin Pomeroy)