Syrian security forces are engaged in fierce clashes with rebels on the eastern outskirts of the capital Damascus, residents say.
Artillery shells and rockets landed inside the heart of the city as part of a surprise rebel attack, they say.
The insurgents detonated two suicide car bombs in Jobar district before trying to storm government defence lines, observers said.
The military responded to the attack with air strikes.
Syrian state media says secret tunnels were also used to launch the assault in Jobar.
Damascus only has a few opposition held areas, and Jobar is the closest to the city centre. Control of the war-damaged area – which is split between rebels and jihadists on one side and government forces on the other – has been fought over for more than two years.
AFP correspondents in Damascus said the army has closed routes into the strategically important Abbasid Square as explosions reverberated across the city.
The UK-based Syrian Observatory for Human Rights said the rebels had initiated the attack to relieve pressure on fighters under attack from government forces in the districts of Barzeh, Tishreen and Qabun.
Last Wednesday, at least 31 people were killed in a suicide bomb attack at the main court complex in the centre of Damascus.
Later, another suicide bomber attacked a restaurant in the western district of Rabweh, injuring more than 20 people.
The attacks come on the sixth anniversary of the uprising against President Bashar al-Assad.
Syria war: Damascus sees fierce clashes after rebel attack