BEIRUT: Fierce infighting between rival groups in eastern Syria left some 24 fighters dead on Thursday, while government shelling killed at least four teenagers in a town in the country's west, activists said.

The four were killed in the rebel-held town of Rastan, just north of the city of Homs, a day after two car bombs exploded in a government controlled district there, killing 25 and wounding over 100.

It was the latest episode in a relentless cycle of blood and violence that has gripped the country since March 2011, when the uprising against President Bashar Assad's rule began.

Opposition groups including the Local Coordination Committees and the Britain-based Syrian Observatory for Human Rights said the teenagers were killed in a barrage of artillery shells that struck a residential district.

"Bashar Assad and his gangs killed the children. They are slaughtering us, old and young and children," said one man, standing beside the bodies shrouded in white sheets, in a video posted online.

Other footage showed heavy smoke rising behind buildings as the shells hit. The videos were consistent with The Associated Press' reporting on the incident.

More than 150,000 people have been killed in the past three years, opposition activists say.