ISIS has executed more than 3,500 people since last June, since establishing a "caliphate" or "Islamic state" in the territories it controls in Syria and Iraq, said the Syrian Observatory for Human Rights, according to AFP. More than half of the people who have been executed by IS were civilians that did not take part in the fighting, including 103 women and 77 children. In total, 1,945 civilians were executed in the past year and a half.

The reasons for the executions range from homosexuality to collaborating with U.S. forces, and even include charges of spying or desertion against their own members, who account for 415 of the total number of people executed. The count also includes the executions of more than a thousand fighters belonging to opposing rebel factions or regime forces.

More than half of the civilians executed were members of the Sunni tribe al-Sheitaat in the Eastern province of Deir Ezzor, with 700 executions taking place within a span of just two weeks in August 2014, claimed the Syrian Observatory, according to Reuters. The tribe has been in conflict with IS since mid-2014.

The Syrian Observatory for Human Rights is an organization based in the United Kingdom, and is largely run by the founder Rami Abdul Rahman, who relies on a network of around 230 activists on the ground in Syria to obtain information, according to The New York Times. This organization is responsible for calculating most of the figures associated with the conflict in Syria.

Since the conflict in Syria began in 2011, more than a quarter of a million people have been killed, including at least 28,000 civilians who have died in random shootings and mass killings, according to The New York Times. Another 27,000 civilians died in artillery and rocket attacks, and another 18,000 civilians died in air strikes carried out by the Syrian government. Since 2011, more than four million refugees have fled the war-torn country.