Syrian monitoring organisation says group killed 1,175 civilians and 116 foreign fighters who wished to return home.
The Islamic State of Iraq and the Levant (ISIL) has killed 120 of its own members, most of them foreign fighters trying to return home, during the past six months, a Syrian monitoring group has said.
The Syrian Observatory for Human Rights said on Sunday that the group also killed 1,175 civilians, including eight women and four children, in the same period.
Rami Abdulrahman, the head of the Syrian monitoring group, told Reuters news agency that 930 of the civilians were members of the Sheitaat, a Sunni Muslim tribe from eastern Syria that fought Islamic State for control of two oilfields in August.
He said that 116 foreign fighters, who had joined Islamic State but later wanted to return home, were executed in the Syrian provinces of Deir Al-Zor, Raqqa and Hassakeh since November. Four other Islamic State fighters were killed on other charges, Abdulrahman said.
Abdulrahman, who gathers information from all sides of the Syrian conflict, said that the group had also executed 502 soldiers fighting for President Bashar al-Assad and 81 anti-Assad fighters.
The figures cannot independently be verified.
ISIL, an offshoot of al-Qaeda, has released videos of executions of captured enemy fighters, activists and journalists.
It beheaded two US journalists, and one American and two British aid workers this year in attempts to put pressure on a US-led international coalition, which has been bombing its fighters in Syria since September.
The armed group has taken vast parts of Iraq and Syria and declared a caliphate in territory under its control in June.
Since then it has fought the Syrian and Iraqi governments, other armed groups and Kurdish forces.