An explosion killed at least five people in Aleppo and two blasts hit a Damascus highway on Saturday in further signs that rebels fighting to topple President Bashar al-Assad are shifting tactics towards homemade explosives.

The British-based Syrian Observatory for Humans Rights, which monitors the 14-month-old revolt, said the Aleppo blast wrecked a car wash in Tal al-Zarazeer, one of the poorest suburbs of the mostly middle-class northern trading city.

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

'We placed a bomb inside a car,' Ali al-Halabi said, naming the car wash owner and accusing him of raping a woman in front of her husband. 'I went to the area afterwards and saw seven bodies and many wounded.'

The Observatory put the death toll at five.

Aleppo has been spared the worst of a conflict that has turned some cities into battle zones, but on Thursday security forces and students wielding knives attacked anti-Assad protesters at the university, killing four and detaining 200.

In Damascus, two bombs exploded on central al-Thawra Street, destroying nine cars. There was no word on casualties.

Reuters journalists saw mangled mini buses and a smashed yellow taxi being dragged away from the area later.

'We heard a big explosion,' said a resident of the nearby Souq Sarouja neighborhood. 'The security forces have blocked off the area now.'

The bombings dealt another blow to a 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 12, 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.

Deadly blasts have shaken major cities as insurgents seek to even the odds between their outgunned forces and the tanks, artillery and helicopters in Assad's military arsenal.

'We want to show foreign journalists in Syria that Damascus is not silent, to embarrass the regime,' said Emad, an activist who lives near al-Thawra Street, giving only his first name for fear of arrest.

On April 30, explosions blew the fronts off buildings in the northern town of Idlib, where state TV reported nine people killed and 100 wounded, including security personnel.

Three days earlier, a suicide bomber killed nine, including security men, at a Damascus mosque, the Interior Ministry said.

An Islamist group calling itself the Support Front for the People of the Levant claimed responsibility for that bombing and for an April 24 attack on the Iranian cultural consulate in Damascus. Iran is one of Syria's closest allies.

Assad has long argued that he is combating foreign-backed 'armed terrorist groups' rather than a popular uprising. Syrian officials say rebels have killed more than 2,600 soldiers and police.

The United Nations estimates that the security forces have killed more than 9,000 people since the rebellion began.-Reuters