Two bombs detonated on a central Damascus road on Saturday (NZ time), in a further sign that rebels fighting to topple President Bashar al-Assad are shifting tactics towards homemade explosives.
There were no immediate reports of casualties from the blasts from bombs planted under cars in the latest blow to last month's crumbling U.N.-backed truce.
Fifty out of a planned total of 300 United Nations observers are now in Syria to monitor the ceasefire declared on April 12th.
But their presence has not halted the violence in a 14-month-old revolt that began with peaceful pro-democracy protests.
The United Nations estimates that the security forces have killed more than 9000 people since the rebellion began.
Elsewhere, activists say a blast on the outskirts of Syria's second city Aleppo has killed or wounded several people.
The blast targeted a bus, according to the opposition Syrian Observatory of Human Rights.
At least four people were killed in the city yesteday when security forces fired on protesting students.