An explosion occurred on Friday in an industrial zone of the Syrian capital Damascus, near a state-owned transport company, the Syrian Observatory for Human Rights said.
The Britain-based watchdog said it was unclear whether there were any victims.
Anti-regime activists said in messages posted on Facebook that a white Mercedes vehicle had exploded in the industrial zone and that there were casualties.
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They said ambulances rushed to the site, which was cordoned off by security.
The blast took place amid a putative UN-backed ceasefire that technically went into effect on April 12 but has failed to take hold, with violence and casualties reported on a daily basis.
The Observatory also said three security agents were wounded on Friday in a blast in the coastal city of Banias. It gave no further details.
More than 9,000 people have died since a popular uprising erupted against the regime of Bashar al-Assad in March 2011, the UN says, while non-governmental groups put the figure at more than 11,100.
