The Syrian Observatory for Human Rights, a Britain-based group monitoring Syria’s conflict, said it had documented 3,027 executions by IS since June 29, 2014.
Among those executed are 1,787 civilians, including 74 children, said the Observatory.
Members of Sunni Shaitat tribe account for around half of the civilians murdered.
IS killed 930 members of the clan in Deir Ezzor last year after they rose up against the extremist Sunni Muslim group.
The toll also includes recent mass killings by IS in the Syrian Kurdish town of Kobane, which the jihadist group re-entered briefly this week after being expelled in January.
The monitor said it had counted at least 223 executions in the border town this week.
The Observatory also documented 216 IS executions of rival rebel factions and Kurdish fighters, as well as the executions of nearly 900 regime forces.
IS has also executed 143 of its own members it accused of crimes including spying, many of them captured as they were trying to desert the group, the Observatory said.
And at least 8,000 IS militants have been killed in battles and US-led air strikes, added the monitor.
IS emerged in Syria in 2013, growing from Al-Qaeda’s one-time Iraq affiliate and initially seeking to merge with Al-Qaeda’s Syrian affiliate Al-Nusra Front.
When Al-Nusra refused the merger, the two groups become rivals, and IS went on to announce its “caliphate” in territory in Syria and Iraq last year, proclaiming its leader Abu Bakr al-Baghdadi “Caliph Ibrahim”. -AFP