The bloody fights erupted to the east of the Syrian Capital, in an area called Eastern Ghouta. It is a densely populated cluster of towns and villages that has been a battlefield since 2011, when the civilian war began.
Local residents and officials tried to prevent bloodshed, calling the sides to start talks in a series of protests. Despite that, battling sides continued to fight, constructing fortifications and blocking roads, the monitoring group added.
"More than 300 fighters have been killed as Islamist rebel factions battle for influence in Eastern Ghouta," SOHR director Rami Abdel Rahman told AFP, specifying that most of casualties befell Jaish al-Islam and al-Nusra factions.
Rahman added that clashes claimed the lives of ten civilians, including a child and a doctor. The doctor, Nabil al-Daas, was the last gynecologist working in the area.
Jaish al-Islam is considered the major rebel faction in the area. One of its leaders, Mohammad Alloush, has presented rebels in Geneva talks on Syrian ceasefire, despite the legitimate government labeling him a terrorist.