Syrian state TV is reporting an explosion in a southern neighborhood of the capital, Damascus.
It says the Sunday morning blast, which happened during rush hour on the first working day of the week, appears to have been "a terrorist act".
The UK-based Syrian Observatory for Human Rights told AFP news agency that the explosion struck near a military intelligence office in Damascus.
"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 monitor said in a statement.
It was unclear if the blast was caused by a bomb that was planted or a suicide attack, the monitor said, adding that shooting followed the explosion.
State media say all roads leading to the scene of the blast have been closed.
Attacks have been rare in Damascus since Syrian government forces captured the last rebel-held neighbourhoods and suburbs of the capital last year.
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 now control almost two-thirds of the country.
The Syrian government in May reclaimed a final scrap of territory held by the Islamic State of Iraq and the Levant (ISIL, also known as ISIS) group in southern Damascus, cementing total control over the capital for the first time in six years.