BEIRUT - Eight rebel fighters have been crucified in Syria by the Islamic State in Iraq and Syria (ISIS) because they were considered too moderate, a monitoring group said on Sunday.

The Syrian Observatory for Human Rights, which relies on contacts on the ground in Syria, said the men were crucified on Saturday in Aleppo province. It added that their corpses were still on view.

The Observatory said clashes between rival Islamist groups in Syria had killed around 7,000 people since January, as militants from ISIS try to strengthen their grip on territory.

The infighting has complicated the insurgency and drawn in foreign fighters.

ISIS, a radical al-Qaida offshoot group, has captured areas on both sides of the Iraqi-Syrian border after seizing the Iraqi city of Mosul on June 10 and sweeping towards Baghdad.

In Syria, ISIS has battled with groups such as the Nusra Front, al-Qaida's official Syrian wing, in the uprising against President Bashar Assad which started with pro-democracy protests but has descended into civil war.

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