DAMASCUS, Syria (Reuters) — An explosion killed at least three 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 toward homemade explosives.

Syria’s state news agency said three people had been killed, one of them a child, and 21 had been wounded by a booby-trapped car in the northern city of Aleppo. The Syrian Observatory for Humans Rights, an opposition group based in Britain that relies on information from Syrian activists, said the blast destroyed a carwash in Tal al-Zarazeer, a poor suburb, and killed five people.

A member of the rebel Free Syrian Army claimed responsibility for the bombing, saying that the carwash was used by members of a pro-Assad militia. “We placed a bomb inside a car,” said the rebel, Ali al-Halabi. He said seven people were killed.

Aleppo, Syria’s largest city and its commercial capital, 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.