* Heavy fighting continues on outskirts of capital
* Assad troops try to "cleanse" rebels from stronghold
* Opposition campaigner says rebels from south join the battle
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, a major opposition centre of the 20-month revolt situated amid farmland 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, and 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, and an air defence position near the southern gate of the capital, according to activists, video footage and diplomats following the military situation.
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, with Britain becoming the ninth country to grant it such status.
NO LONGER A STALEMATE?
Shashank Joshi of the Royal United Services Institute (RUSI) in London said the developments of the last few weeks were shifting the balance in favour of the rebels.
"The use of the world 'stalemate' to describe the conflict may no longer be appropriate," he told Reuters by phone. "The rebels have moved up the ladder of warfare."
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.
"The whole eastern Ghouta is basically a liberated area. Assad's army still has superior firepower, but is being eroded. It can no longer push forward with a lot of troops," said Abu Ghazi, an activist-turned-fighter in Irbin.
Severe restrictions on non-state media make it impossible to verify such reports independently.
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.
But there were suggestions that the latest fight for the suburb might be following a different course.
Live footage broadcast by the opposition on the Internet showed heavy smoke rising from a built-up area in Daraya and carried the sound of automatic gun fire.
"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.
CHANGE ON THE GROUND
Seven civilians and three rebels were killed in fighting and bombardments on Daraya, opposition sources said.
Two 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.
The official state news agency said that "terrorists" - a term it uses for rebels - had attacked shops and homes in Daraya, as well as a mosque.
"Last time the rebels were in Daraya, they worked separately and the regime moved in, drove them out and took revenge on the civilian population," said Fawaz Tello, a veteran opposition campaigner with links to rebels.
"The fact that the rebels have recaptured Daraya and are fending off Assad's best forces indicates a change on the ground," Tello said from Berlin. "The rebels' military position is still difficult, but it is improving."
So far Assad's core military units, composed mainly of members of his Alawite minority sect, an offshoot of Shi'ite Islam, have prevented a rebel push into the capital itself.
But Tello said the rebels were gaining strength in Damascus, partly because they were being joined by fighters from outlying regions, especially the southerly Hauran Plain, birthplace of the revolt.
He pointed to guerrilla attacks in the last few days in Hetaytet al-Turkman, near Damascus Airport, and expanding rebel control of the mixed urban and farmland regions around Damascus, although Assad's forces controlled the main road junctions.
RUSI's Joshi said anti-aircraft weapons looted from military bases would blunt what is the government's most important weapon: air power.
These advances, he said, "are all a symptom of tactical improvements. The more they fight, the better they get".