An explosion hit a security forces' bus in Damascus on Sunday and wounded several people, activists said.

Residents said they heard a powerful blast, followed by the sirens of ambulances rushing toward Damascus's southern ring road near the neighbourhood of Midan.

Some activists said more than one member of the security forces was killed in the attack, but others said there were no dead, only wounded.

They said the blast appeared to have been caused by an improvised explosive device that had been stuck onto the bus.

There was no immediate comment from the government.