Syrian raids have been targeting rebel forces in the eastern outskirts of Damascus [AFP]
Syrian government jets bombarded opposition-held neighbourhoods of Damascus on Monday a day after opposition forces launched a surprise assault.
Rebel groups allied with former al-Qaeda affiliate Jabhat Fateh al-Sham attacked government positions in the east of the capital early on Sunday morning in a complex and co-ordinated operation.
But forces loyal to President Bashar al-Assad drove them back by nightfall, a war monitor said, and began a fierce bombing campaign on Monday morning.
"There have been intense air strikes since dawn on opposition-held positions in Jobar from which the offensive was launched," Rami Abdel Rahman, head of the Britain-based Syrian Observatory for Human Rights, said.
"The government and allied forces have retaken the initiative and are striking the groups that launched yesterday's assault," he added.
Abdel Rahman said it was unclear whether government forces or their Russian allies were carrying out Monday's raids on Jobar.
The fighting killed at least 26 government soldiers or allied fighters and 21 rebels, Abdel Rahman said, but he did not have an immediate toll for Monday morning's raids.
Al Jazeera's Mohamed Al Jazaeri, reporting from near Damascus, said at least 15 civilians had been killed after government forces shelled residential neighborhoods in Eastern Ghouta.
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".
"We understand that since daybreak there has been around 30 airstrikes in [Jobar]," Tyab said, adding that there were reports of heavy rebel losses, although credible figures were not yet available.
'Opposition remains powerful'
On Sunday, opposition fighters seized several buildings in Jobar before advancing into the neighbouring Abbasiyn Square area - the first time in two years that the opposition advanced so close to the capital's centre.
Fighting raged close to the old city in Damascus and the advance linked Jober to Qaboun and other areas to the north which had been under government siege.
Control of Jobar - which has been a battleground for more than two years - is divided between rebels and allied fighters on one side, and government forces on the other.
Tahrir al-Sham - an umbrella group of hardline rebels formed by Jabhat Fateh al-Sham last month - and the independent Failaq al-Rahman group also participated in the attack.
Tyab said the timing of the rebel offensive was signficant, marking six years since the start of the Syrian uprising.
"I think it was really was in the calculus of the rebels that they want to make it known that they are still going to resist the regime, that they are still going to fight, despite the very heavy losses that they have experienced," he said.
Tyab reported that rebel fighters used car bombs, suicide bombs, and tunnels in their assault.
"We understand that some of these [rebel] fighters have been able to launch mortar shells inside the city itself."
Syrian state TV on Monday quoted Russia's ambassador to Damascus as saying one of the embassy's buildings was hit with a shell during the clashes.
Joshua Landis, an expert on Syria at the University of Oklahoma, told Al Jazeera on Monday that the rebel offensive had taken the government by surprise and that a significant response was inevitable.
"The regime is going to realise that it cannot allow these two areas to linger there because they are beachheads for this Tahrir al-Sham group to make inroads into the Damascus area," he said, adding the government would likely withdraw some forces from areas such as Homs and Hama to refocus on Damascus.
"It means that the fight is still on, there are many fronts to this war, and the opposition remains powerful."