Syria's opposition fighters battled government forces in Damascus into the early hours of Monday, in what residents have described as the fiercest fighting yet inside the capital.
On Sunday social media websites published video which purportedly show smoke rising from the Al Tadamon and Naher Aisha neighbourhoods of the Syrian capital, as a result of shelling by pro-government forces.
Activists say fighting was under way in the poor al-Tadamon district, after a night of sustained battles in the nearby Hajar al-Aswad district.
There were reports of heavy gunfire and smoke rising in the area and wounded people and residents trying to flee.
Armoured vehicles were reportedly heading towards the southern part of the neighbourhood.
The fighting came as UN and Arab League envoy Kofi Annan prepared to meet Russia's foreign minister for talks on the Syrian crisis.
Since the conflict began, Russia has been a staunch supporter of Syrian leader Bashar Al Assad.