"A terrorist suicide explosion... hit the Rukn al-Din neighbourhood of Damascus. There are reports of victims," state television said although it did not provide a toll.
The Syrian Observatory for Human Rights said three people killed in the explosion, but it did not specify what had caused the blast.
The Britain-based group also said security forces deployed en masse in the district and closed off several roads in the area.
Rukn al-Din adjoins the flashpoint Barzeh district, scene of fierce fighting pitting rebels against troops after the army recently escalated its efforts to crush the insurgency around Damascus.
Regime forces shelled Barzeh on Sunday and clashed with rebels, the Observatory said, adding that there were several casualties.
It also reported that several shells fell near a security branch in the Bab Musalla district of southwestern Damascus.
Violence also raged in the Damascus countryside.
The army battled rebels both in the Eastern Ghouta area east of the capital and in the Qalamoun area that lies between Damascus and the Lebanese border,said the Observatory.
In Qalamoun, at least six rebels were killed in Sunday morning`s fighting, the group added.
In northern Syria, meanwhile, 12 troops were killed in a rebel car bomb attack at the edges of Aleppo city, said the Observatory.
Clashes also rocked the New Aleppo district in the western part of the northern city on Sunday, a day after rebels said they launched an offensive against loyalist forces, the Observatory said.
Regime aircraft shelled the flashpoint neighbourhood, it added.
A powerful explosion, meanwhile, rocked Aleppo`s Minnigh air base, which rebels fighting to topple President Bashar al-Assad`s regime have been trying for months to wrest from army control.
Syria`s 27-month war has left more than 93,000 people dead, according to the United Nations.