At least five people have been killed in a bomb blast in Syria's second city Aleppo, according to opposition activists.

The British-based Syrian Observatory for Humans Rights said five people were killed in the Aleppo explosion, which wrecked a car wash in Tal al-Zarazeer, one of the poorest suburbs of the city.

A member of the rebel Free Syrian Army claimed responsibility for the bombing, telling Reuters that the car wash was used by members of a pro-Assad militia.

"We placed a bomb inside a car," Ali al-Halabi said. "I went to the area afterwards and saw seven bodies and many wounded."

The blast follows an attack on anti-Assad protesters at Aleppo university on Thursday by security forces and knife-wielding students in which four people were killed and 200 were detained.

In Damascus, two bombs exploded on al-Thawra Street on Saturday, destroying nine cars.

No one has yet claimed responsibility for the explosions and no casualties have so far been reported.

The bombings are another blow to the fragile UN-backed ceasefire between government and rebel forces. Fifty out of a planned total of 300 UN observers are now in Syria to monitor the truce, but their presence has not halted the violence.

Activists said at least 37 people were killed on Friday when security forces fired on protesters around the country.