The UK-based Syrian Observatory for Human Rights (SOHR) on Sunday accused Daesh of murdering nearly 2,000 Syrians since announcing their "caliphate" in June.
"The SOHR has documented the Daesh execution of 1,878 people in Syria since June 28 when it announced its 'caliphate'," the group said in a statement published on its official site.
It said the victims were shot dead, beheaded or stoned to death in the provinces of Aleppo, Deir Ezzor, Hama, Homs, Hasakeh and Raqa.
Of those killed, 1,175 were civilians who included four children and eight women.
The dead included 930 members of the Shaitat tribe which rose up against IS in the eastern province of Deir Ezzor in the summer.
On December 17, the Observatory said a mass grave containing the bodies of 230 Shaitat had been found in the province.
The jihadists also "executed" 502 soldiers and pro-regime militiamen, the monitoring group said.
Daesh also killed around 120 of its own members, mostly for trying to flee to their home countries, and 80 members of the rival An-Nusra Front, the Al-Qaeda affiliate in Syria.