A firefight is reported to have erupted in Syria's capital, Damascus, between the rebel Free Syria Army and the forces of President Bashar al-Assad.
Witnesses say the sound of machine-guns and rocket-propelled grenades can be heard from the district of al-Mezze.
The neighbourhood, in the centre of Damascus, is home to several security facilities and is one of the most heavily guarded parts of the capital.
In January the Free Syria Army briefly seized several Damascus suburbs.
Al-Mezze has been the scene of large anti-government protests.
"There is fighting near Hamada supermarket and the sound of explosions there and elsewhere in the neighbourhood," a resident told Reuters news agency.
"Security police have blocked several side streets and the street lighting has been cut off."
On Sunday a car bomb exploded in the northern Syrian city of Aleppo, killing at least two people and injuring 30 others.
A day earlier, at least 27 people were reported to have been killed and 97 wounded in two explosions in the capital.
State TV described the blasts as "terrorist" attacks.
However, activists have accused the authorities of staging incidents to discredit opposition groups.
President Assad is trying to quell an increasingly armed rebellion that sprang from a fierce crackdown on peaceful pro-democracy protests a year ago.
The UN estimates that more than 8,000 people have died in the clashes.
President Assad insists his troops are fighting "armed gangs" seeking to destabilise Syria.