Syrian forces and rebels battled on the fringes of Damascus Sunday after the insurgents infiltrated the capital through tunnels overnight and staged a surprise assault on government-held parts of the city.
Earlier, state media said the military had recaptured all of the territory it had lost in Sunday's rebel assault.
The Syrian Observatory for Human Rights, a UK-based monitoring group, said pro-government fighters and 21 rebels and jihadists were killed in the fighting.
Fighters tried to make their way to the city center, trading fire with government forces that could be heard across Damascus.
Syrian state TV aired footage from Abbasiyn Square, typically buzzing with activity but now empty except for the sound of shelling.
Jabhat Fateh al-Sham was formerly known as al-Nusra Front before purportedly breaking away from the outfit, which operated as al-Qaeda's Syria branch.
The government has been trying to pressure the rebels to surrender the pockets they hold in Damascus, following victories in the northern city of Aleppo, the central city of Homs and other Damascus suburbs.
"These are not intermittent clashes - these are ongoing attempts to advance", he said.
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Roseline Ebhoramen reports that, artillery shells and rockets landed in the heart of the capital as part of a surprise rebel attack launched in the Jobar district.
Al Jazeera's Imtiaz Tyab, reporting from Beirut, said people in the old city reported walls reverberating with the ferocity of the government's bombardment, and that rebel activists had described the raids as "relentless".
The fighting comes only days before another round of UN-brokered peace talks in Geneva aimed at finding a political solution to Syria's six-year civil war.
The latest skirmishes were concentrated in the zones surrounding Barza and Qaboun, which were cut off by the army from the remainder of the major rebel enclave of Eastern Ghouta and the districts in the eastern side of Damascus.
The Observatory said fighting was still underway Monday.
The army and allied militias have been targeting the besieged Eastern Ghouta area, the biggest remaining rebel bastion around the capital, for months, making incremental gains.
"They're spread too thin and this has allowed rebels to advance".