A human rights group estimates the number of people executed by the extremist group Islamic State since June, at 2,000 including some of its own fighters.

The UK-based Syrian Observatory for Human Rights said in a statement released Sunday that it has documented the execution of around 1,200 civilians, mostly in the Syrian provinces of Deir al-Zour, Al Raqqa, Hama, Aleppo, and al-Hasaka.

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The monitoring group also reported that the IS has killed 120 of its own fighters, mostly foreign extremists who had joined its ranks but have decided to flee back to their home countries. Four other fighters were said to have been killed for violating the group's strict code.

The observatory said the IS, which is seeking to establish a caliphate within the borders of Syria and Iraq, has executed some 500 Syrian soldiers fighting for President Bashar al-Assad and approximately 100 members of other anti-Assad groups fighting in Syria.

The head of the observatory, Rami Abdulrahman, told Reuters that 930 of the civilians were members of a Sunni Muslim tribe which fought the IS over two oilfields in eastern Syria.

Since the start of the extremists' path of destruction, they have used public executions to tighten their grip on communities, even using social media to broadcast beheadings.

This year, the IS beheaded two US journalists and two aid workers, in response to bombing raids by a US-led international force.   

More than 60 countries have joined the coalition fighting the Islamic State, according to the US State Department in November, with the US and its closest allies doing the bombings and providing air support to ground forces. Some countries offer delivery services while others are providing humanitarian aid to the countless Syrians and Iraqis displaced by the conflict.

Also on Sunday, the US Central Command reported the destruction of 13 IS fighting positions and a large IS unit by eight coalition airstrike operations in the Kobani area in northern Syria.

TagsExecuted, rights group, IS, US coalition, Iraq