BEIRUT (Reuters) - Fighting between insurgent groups east of Damascus killed more than 50 people on Monday, bringing to more than 500 the number of people killed in rebel infighting since late April, the Syrian Observatory for Human Rights reported.

The fighting pits Jaish al-Islam, which is part of the opposition High Negotiations Committee (HNC), against rival faction Failaq al-Rahman, and is threatening Jaish al-Islam’s dominant position in Eastern Ghouta just outside Damascus.

Jaish al-Islam, which runs the civil administration in the region and has some 7,000 fighters there, suffered a blow in December when its leader, Zahran Alloush, was killed in an air strike.

Failaq al-Rahman controls several towns slightly closer to Damascus, also in Eastern Ghouta, with about 2,000 fighters. Its allies in this particular fight include Islamists, some al Qaeda-linked, but the group itself is more moderate.

Some of the more moderate rebel groups have fought alongside al Qaeda-affiliated Nusra Front during Syria’s five-year civil war because of the group’s strength on the battlefield against government forces and their allies.

In a statement on Monday, Jaish al-Islam accused Failaq al-Rahman of rejecting an initiative tabled by HNC coordinator Riad Hijab to end the bloodshed in Eastern Ghouta.

A statement by a local body which the Observatory said was close to Failaq al-Rahman on Tuesday criticised Jaish al-Islam for failing to withdraw from a town in the area, under an agreement reached between the two sides a week ago.

The statement said Jaish al-Islam had launched a new assault nearby on Tuesday.

Several attempts at ending the fighting have failed.

Jaish al-Islam has accused Failaq al-Rahman of wrecking attempts to stop the fighting by launching its own attacks.

The clashes began on April 28, when Jaish al-Islam’s positions in the Eastern Ghouta came under attack, the Observatory said. It said nine civilians including three children had been killed in the fighting since then.

Failaq al-Rahman has been backed in the fighting by a group known as the Fustat Army, whose fighters include members of the Nusra Front, the British-based Observatory says.

Each side has captured dozens of fighters, it said.

During the rebel infighting, government forces have attacked some areas of Eastern Ghouta and made advances there, the Observatory has reported.

The fighting between the insurgent groups in Eastern Ghouta is one of many smaller-scale battles taking place in the many-sided Syrian conflict, which has killed more than 250,000 people and is in its sixth year.

(The story was refiled to correct the name of the rebel group in paragraph 10 to Failaq al-Rahman)