BEIRUT -- Up to 7,000 people, mostly rebels fighting to overthrow President Bashar Assad, have been killed in infighting among rival Islamic groups in Syria across opposition-held territory in the north, an activist group said in a report Sunday.

The Britain-based Syrian Observatory for Human Rights says it has documented 7,000 deaths as a result of the rebel-on-rebel violence since January, when infighting erupted in northern Syria. The death toll also included 650 civilians who got caught in the crossfire of the fighting between the al Qaeda-linked Nusra Front and its rival, the Islamic State of Iraq and the Levant - a group which formally broke with al Qaeda earlier this year and has in recent weeks become a major fighting force in neighboring Iraq.

The Observatory has been documenting the Syrian conflict through a network of activists inside Syria since it started in March 2011 as largely peaceful protest against Assad's rule. It turned into an armed uprising after some opposition supporters picked up arms to fight a brutal government crackdown on dissent. It gradually became a civil war, in which more than 160,000 people have been killed, according to activists, and nearly a third of Syria's population of 23 million has been displaced.

In Sunday's report, the Observatory said its activists on the ground have the names of 5,641 rebels who have been killed in infighting. The names of another 1,200 dead fighters have not been confirmed. Up to 2,196 fighters who have been killed in clashes, suicide bombings and other rival attacks belonged to the Islamic State, while 2,764 were killed on the side of the al-Qaida-linked Nusra Front and other Islamic groups fighting alongside it, the Observatory also said. The remainder of the dead were members of other groups.

The two rebel factions have been engaged in deadly infighting in opposition-held territory in several provinces in northern and eastern Syria, along the border with Turkey and Iraq, including Aleppo, Raqqa, Hassakeh and the oil-rich province of Deir el-Zour. The infighting over territory and strategic facilities - including oil-fields - that rebel groups captured together from government forces, has undermined the rebels' larger goal of toppling Assad.

The Syrian leader secured a third, seven-year mandate at a presidential election earlier this month.