It was the deadliest attack in Damascus in several years, and especially rare since government troops in 2018 captured suburbs once held by insurgents in Syria's decade-long conflict.

State media had earlier described the attack as a roadside bombing.

But an unnamed Syrian military official said the explosions were caused by bombs that had been earlier attached to the vehicle's exterior. A third bomb fell from the bus and was dismantled by troops after the two initial explosions, the official said.

No one immediately claimed responsibility for the attack, but insurgents and jihadists still reside within the country's borders and seek the overthrow of President Bashar Assad.

It was unclear if the bombs were detonated from a distance or if they had been timed to go off. The military official, quoted in state media, said the bombs went off shortly before 7am on Thursday.

The explosion went off at a busy junction in the Syrian capital, near a main bus transfer point under a bridge known as President's Bridge. Commuters converge and head out to different parts of the capital and its suburbs from the area.

Syrian state TV showed footage of the charred bus, saying the blasts occurred as people headed to work and school.

"It is a cowardly act," Damascus police commander Major General Hussein Jumaa told state TV, adding that police had cordoned off the area immediately and made sure there were no more bombs.

Jumaa said one of the wounded had succumbed to his wounds later on Wednesday raising the death toll to 14.

Australian Associated Press