The Syrian Observatory for Human Rights, a Britain-based group monitoring Syria’s conflict, said it had documented 3,027 executions by ISIS 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.
ISIS killed 930 members of the clan in Deir Ezzor last year after they rose up against the extremist group.
The toll also includes recent mass killings by ISIS in the Syrian Kurdish town of Kobane, which the terrorists 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 ISIS executions of rival rebel factions and Kurdish fighters.
ISIS 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.