Death toll from fierce infighting between rival Islamic rebel groups in eastern Syria has risen to at least 68 fighters, an opposition group said, while government shelling left at least four teenagers dead in a town in the country's west.

The Britain-based Syrian Observatory for Human Rights said battles were still raging on Friday, for a second day, around Bukamal town in the oil-rich Deir Ez Zor province near the Iraqi border.

Rebels from the al-Qaeda breakaway group known as the Islamic State of Iraq and the Levant and fighters of the al-Qaeda-linked Nusra Front and other groups have been fighting each other in the province for weeks over territory they previously captured together from President Bashar Assad's forces, including oil-fields.

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 Assad's rule began.

Syria's conflict began three years ago with largely peaceful protests calling for reform, and later for Assad to be toppled.

Al-Qaeda-linked goups, with foreign fighters in their ranks, have played an increasingly prominent role, dampening support from the West.

Thousands of fighters have been killed in rebel-against-rebel violence that intensified beginning of the year, particularly in northern and eastern Syria.

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

The latest violence came a day after activists said four teenagers were killed in the rebel-held town of Rastan, just north of the city of Homs.

Opposition groups including the Local Coordination Committees and the Observatory said a barrage of artillery shells killed the teenagers.