A "huge explosion" near a military intelligence office in Damascus on Sunday left a number of dead and wounded, a war monitor said.
"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.
Syria is locked in a war that has killed more than 360,000 people and displaced millions since a crackdown on anti-government protests in 2011 spiralled into armed conflict.
With key military backing from Russia, President Bashar Al Assad's forces have retaken large parts of Syria from rebels and militants, and now control almost two thirds of the country.
The Syrian regime in May reclaimed a final scrap of territory held by ISIS in southern Damascus, cementing its 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 more than a year.
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