A car bomb in the northern Syrian city of Aleppo has killed at least five people, according to the Britain-based Syrian Observatory for Human Rights.
The group relies on activists inside Syria for information.
A separate bomb in Damascus damaged nine cars, but there were no reports of any casualties.
No-one immediately claimed responsiblity for either explosion.
The bombings are the latest blow to a peace deal struck Kofi Annan last month.
Ahmad Fawzi, a spokesman for the former UN secretary general, said progress is being made.
“There are no big signs of compliance on the ground. There are small signs of compliance. Some heavy weapons have been withdrawn, some heavy weapons remain, some violence has receded,” he told reporters in Geneva.
Under the Annan plan, a team of 300 observers will monitor the Syrian government’s compliance with the truce.
The 14-month uprising against President Bashar Assad’s rule has left more than nine thousand civilians dead, according to the UN.