President Bashar al-Assad's forces have bombarded south-east Damascus with air strikes and artillery in an attempt to dislodge rebel fighters.
A Middle East diplomat described battles in the city as a "major engagement", with fighting going back and forth. He said: "The opposition is hitting Damascus from a multiple of directions and the regime is trying to stop it."
Jets bombed Jobar, a neighbourhood adjacent to the main Abbasid Square, and the suburb of Daraya on the highway to Jordan to the south, sources said.
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The two areas are part of interconnected Sunni Muslim districts in and around Damascus that have been at the forefront of the 22-month uprising against four decades of Assad family rule.
Rebels entered Jobar last week after breaching the regime's defence lines at the ring road and taking several army and pro-Assad militia positions in the district.
The road, a supply line for elite army units dug in around the city centre, separates the capital from the mostly rebel-held expanse of Sunni towns and suburbs known as eastern Ghouta.
To the south-west, near the main highway to Jordan, heavy bombardment was reported on the suburb of Daraya, where the army advanced in the last few days, breaking a two-month rebel hold.