The reports came as President Bashar Assad's forces pressed on with a major offensive in the suburbs against opposition fighters who have been closing in on parts of the Syrian capital. To the north, regime troops surged around the contested town of Qusair in Syria's Homs province, near the frontier with Lebanon.
The precise number of those killed in the latest fighting in Jdaidet Artouz and Jdaidet al-Fadel suburbs could not be immediately confirmed.
The Britain-based Syrian Observatory for Human Rights said the number of the dead could be as high as 250. Rami Abdul-Rahman, who heads the Observatory, said the group has documented 80 names of those killed but fears a much higher toll.
The Local Coordination Committees, another activist group, said the death toll was 483 and that most of the victims were killed in Jdaidet Artouz. State-run news agency SANA said Syrian troops "inflicted heavy losses" on the rebels in the suburbs.
Conflicting reports of death tolls are common in Syria's crisis, especially in areas that are difficult to access because of the fighting. The government also bars many foreign journalists from covering the conflict. Both activist groups, the Observatory and the LCC, rely on a network of activists on the ground in different parts of Syria.
Also Monday, two bombings targeted an army checkpoint and a military post in a third Damascus suburb, Mleiha, killing eight soldiers there, according to the Observatory.
Over the past two weeks, the Syrian military, supported by the Hezbollah-backed militia known as the Popular Committees, has pushed to regain control of the border area. The region is strategic because it links Damascus with the Mediterranean coastal enclave that is the heartland of Assad's Alawite sect, an offshoot of Shiite Islam.
The fighting around Qusair also points to the sectarian nature of the Syrian conflict, which pits a government dominated by the president's Alawite minority against a primarily Sunni Muslim rebellion, and underscores widely held fears that the civil war could drag in neighbouring states.
The pro-government daily Al-Watan predicted Monday that "the liberation" of the Qusair area will be completed within a "few days." Troops have already captured several towns and villages around the town.
The report claimed the army was making a "rapid" advance in the outskirts of Qusair, inflicting heavy losses on the rebels and forcing some of them to retreat toward Lebanon.
In Lebanon, there are deep divisions over the Syrian conflict, with Lebanese Sunnis mostly backing the opposition while Shiites support Assad. Lebanese fighters have also travelled to Syria to join either Sunni or Shiite groups, and several have been killed in clashes.
Over the weekend, several rockets fell in the predominantly Shiite Lebanese towns and villages along the border and some Lebanese schools in the area remained closed Monday for fear of more shelling.
Syria's conflict started with largely peaceful protests against Assad's regime in March 2011 but eventually turned into a civil war. More than 70,000 people have been killed so far, according to the United Nations.