In the last month alone IS executed 53 people -- including 35 civilians -- in areas it controls in Syria, the Syrian Observatory for Human Rights said.
The new figures from the Britain-based Observatory bring to 3,591 the number of people executed by IS in Syria since it declared its Islamic "caliphate" after seizing control of large parts of the country.
The new toll includes 1,945 civilians, including 103 women and 77 children, with civilians defined as those who are not taking part in the fighting.
Some were executed for alleged witchcraft, homosexuality and collaborating with the US-led coalition that has been bombarding IS in Syria since September 2014.
Members of Sunni Shaitat tribe account for around half of the civilians killed. IS killed 930 members of the clan in the northeastern Deir Ezzor province last year after they opposed the extremist Sunni Muslim group.
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The monitor also documented 247 IS executions of rival rebels and Kurdish fighters, as well as the executions of 975 members of regime forces.
IS has also executed 415 of its own members it accused of crimes including spying, many of them captured as they were trying to desert the group, according to the Observatory, which relies on a network of activists, medical staff and fighters on the ground.
IS controls large swathes of Syria and neighbouring Iraq.
In Syria, it controls areas in the Homs and Hama provinces in the centre, Deir Ezzor and Hassakeh in the northeast, as well as Raqa and Aleppo in the north.
The jihadist group counts tens of thousands of fighters and carries out abductions, rapes, beheadings and stonings in the areas under its influence.
The United Nations has accused it of "crimes against humanity".
More than 250,000 people have been killed and more than four million have fled the country since the Syrian conflict erupted in 2011.