Fierce infighting between rival Islamic rebel groups in eastern Syria killed more than 50 fighters Thursday, an opposition group said, while government shelling left at least four teenagers dead in a town in the country's west.

The rebel infighting took place around the town of Bukamal in the oil-rich Deir el-Zour province near the Iraqi border between 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 Islamic groups. The two sides have fought each other for months other over territory they previously captured together from President Bashar Assad's forces.

The Britain-based Syrian Observatory for Human Rights said 51 died in the rebel-on-rebel fighting Thursday. The numbers could not be independently confirmed and calls to activists in the area went unanswered.

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's ouster. It has evolved into a civil war and Islamic extremists, including foreign fighters and Syrian rebels who have taken up hard-line al-Qaeda-style ideologies have played an increasingly prominent role among fighters, 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.

Teens killed in bombing

Meanwhile, activists said four teenagers 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.

Two car bombs exploded in Homs on Wednesday, killing at least 25 and injuring more than 100, state media reported. (SANA/AP)

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

UN humanitarian affairs chief Valerie Amos condemned the Wednesday car-bombing in Homs, saying "Two volunteers from the Syrian Arab Red Crescent were among the injured as they arrived in an ambulance to treat people hurt in the first blast, and were caught in the second."

Attacks on civilians are war crimes and may also amount to crimes against humanity," Amos said.

In Damascus, state-run news agency SANA said rebel mortar fire killed two civilians and wounded seven on the city's outskirts.