Unprecedented clashes between troops and rebels raged into a second day in Syria's second city of Aleppo on Saturday, forcing residents to flee some neighborhoods, activists said.

In Damascus, where troops were engaged in a major counter-offensive to retake rebel-held areas on Thursday, activists said the city was largely calm, but remained tense, with clashes continuing in some districts.

The Britain-based Syrian Observatory for Human Rights said in Aleppo "battles have been ongoing since Friday morning between regular forces and fighters from rebel units in the Salaheddin neighborhood."

The Local Coordination Committees — a grassroots activist network — reported "an exodus of residents of the neighborhood because of fear of regime bombardment and an offensive" against Salaheddin.

The fighting is the fiercest yet in the northern commercial hub, which had been largely shielded from the protests and violence that have accompanied the country's 16-month uprising against President Bashar al-Assad.

In recent months, it has seen both fighting and demonstrations, with its university becoming a nerve center for protests against the regime.

In Damascus, the atmosphere was tense after a mass exodus of residents following the heaviest clashes yet in the capital.

Residents said Damascus was largely quiet on Saturday morning, with traffic moving and no sound of clashes in the center of the city overnight.

But the observatory said regime forces had bombarded the Al-Kaddam and Assali neighborhoods on the southern outskirts overnight.

The watchdog reported government forces were shelling several districts of the rebel city of Homs, including Khaldiyeh, and also said fighting was ongoing in the eastern city of Deir az-Zour.

After the Tuesday announcement by rebels of the "battle for the liberation of Damascus," regime forces have stepped up operations in the city, as well as elsewhere in the country.

The observatory said Saturday that death toll in nationwide clashes on Friday stood at 233, including 153 civilians, 43 regime troops and 37 rebel fighters.