International peace envoy Lakhdar Brahimi was due to start talks Monday with Syrian President Bashar al-Assad in Damascus, while heavy clashes engulfed several areas of the capital, activists said.
Brahimi, who arrived in Syria on Sunday, will meet with Foreign Minister Walid al-Moallem and al-Assad, amid reports that his mission has struck a "dead end."
The UN-Arab League envoy's arrival coincided with a government airstrike on a bakery in the central town of Halfaya, in which 109 people were killed, according to activists. More than 50 people were still listed Monday as being in a critical condition.
Activists in Damascus told dpa clashes erupted between rebels and al-Assad's forces in Khaled Ibn al-Waild neighbourhood and lasted throughout the early morning hours.
The Britain-based Syrian Observatory for Human Rights said electricity had been cut throughout the capital since Sunday.
In the northern province of Aleppo, rebels claimed they had taken control of an air force brigade barracks, confiscating "huge amounts of ammunition."
News from Syria is difficult to verify as authorities have barred most foreign media from restive areas since a pro-democracy uprising started in March 2011.