A "huge explosion" near a military intelligence office in Damascus Sunday left a number of dead and wounded, a war monitor said, after state TV reported early indications suggested a "terrorist act".
The Syrian Observatory later confirmed that at least three civilians had been killed in the blast.
"The explosion took place near a security branch in the south of the city. There are some people killed and injured but we could not verify the toll immediately," the Britain-based Syrian Observatory for Human Rights told AFP.
It was unclear if the blast was caused by a bomb that was planted or a suicide attack, according to the monitor, which relies on a network of sources inside the country.
It said that shooting followed the explosion.
Syrian state television earlier reported that a blast had been heard around the southern highway in Damascus.
"First reports suggest a terrorist act," the broadcaster said.
The state outlet did not provide any more details on the incident.
Syria is locked in a civil war that has killed more than 360,000 people and displaced millions since a brutal crackdown on anti-government protests in 2011 spiralled into full conflict.
With key military backing from Russia, President Bashar al-Assad's forces have retaken large parts of Syria from rebels and jihadists, and now control almost two-thirds of the country.
The Syrian regime in May reclaimed a final scrap of territory held by the Islamic State group in southern Damascus, cementing total control over the capital for the first time in six years.
Observatory chief Rami Abdel Rahman said that Sunday's blast appeared to be the first attack in Damascus in over a year.