The Islamic State of Iraq and the Levant (ISIL) group has killed 1,878 people since the declaration of their "caliphate" in Syria on June 28, according to the Syrian Observatory for Human Rights.
The Observatory, which monitors the Syrian conflict, documented the deaths of 1,175 civilians, including four children and eight women.
The execution methods used were beheading and shooting or stoning in the provinces of Deir Ezzor, al-Raqqa, al-Hasakah, Aleppo, Homs and Hama.
ISIL has seized large swathes of land in Syria and neighbouring Iraq and established its headquarters in the city of Raqqa.
In Syria, the group battles both regime forces and rebel groups for control of territory.Since June it has killed 502 officers and soldiers after arresting them during clashes, the Observatory said.
The death toll given by the UK-based group also included 930 members of the Shaitat tribe which revolted against ISIL in the eastern province of Deir Ezzor in the summer.
Earlier this month a mass grave containing the bodies of 230 Shaitat was reportedly found in the province.
The report also said 120 fighters in ISIL's own ranks were killed, most of them foreign fighters trying to return home.
The UN commission of inquiry on Syria released a report in November accusing ISIL of committing war crimes and using terror to "subjugate Syrians living in its areas of control".
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"Those that fled consistently described being subjected to acts that terrorise and aim to silence the population," said Paulo Pinheiro, chair of the commission.
"The commanders of ISIS have acted wilfully, perpetrating these war crimes and crimes against humanity with clear intent of attacking persons with awareness of their civilian or 'hors de combat' [non-combat] status," the report said, using an alternate acronym for ISIL.
The report was based on more than 300 interviews with people who have fled areas under the control of ISIL, as well as photographs and video footage released by ISIL itself.
The group has released videos of executions of captured soldiers and rival fighters, activists and journalists.
The commission called on the perpetrators to be brought to justice, for instance, before the International Criminal Court.
According to the Observatory, more than 200,000 people have been killed since the start of the Syrian conflict that started as an uprising against President Bashar al-Assad's rule.
Source: Al Jazeera