Syrian government forces Saturday escalated their onslaught on rebel-held areas in the city of Aleppo and retook control of a restive area in the capital Damascus, opposition activists said.

In Aleppo, Syria's main commercial centre, rebels said they had foiled two attempts by government forces to recapture the district of Salaheddin south of the northern city and seized two tanks from the army.

"Fierce clashes took place between our fighters and the regime troops near the Syrian television station in Aleppo," Abu Omar al-Halabi, a commander in the rebel Free Syrian Army, told DPA by phone.

He added that rebels had made a "tactical retreat" after government used fighter jets to strafe the area.

Rebels and government forces have been battling in Aleppo for a week now, with neither side making significant gains. The battle could decide the course of the 17-month conflict in Syria.

Elsewhere in Syria, President Bashar Assad's forces have regained most of the district of al-Tadamon in southern Damascus after heavy fighting with rebels, said activists.

Haytham al-Abdullah, an activist, told DPA that Assad's forces had executed several people after entering the district and that at least 12 were killed in the offensive. The claim could not be verified independently.

Government forces have been carrying out house-to-house raids in Damascus since July 18 when a bomb attack killed four top security officials from Assad's inner circle, dealing a severe blow to his regime.

More than 20,000 people have been killed in Syria since a pro-democracy revolt erupted in March last year, according to the opposition.