Seven children were among 15 civilians killed by jihadist rebels in a feud with Kurds in the north-east of Syria, Amnesty International says.
Fighters from the Islamic State of Iraq and the Levant (Isis) targeted families working in a farming community in Hassakeh province in May, it adds.
It followed fighting between Isis and Kurdish armed groups in the area.
The al-Qaeda-linked group is currently battling rival rebels in the north, as well as the Syrian government.
Isis grew out of the former Islamic State of Iraq, a jihadist militant umbrella group that included al-Qaeda in Iraq.
Five men, three women and seven children from two families working on land mainly owned by Yezidi Kurds were killed in the Isis raid on 29 May, the Amnesty report says.
The rights group said that the victims may have been mistaken for Kurds belonging to the Yezidi faith, who had mostly fled the area after Isis fighters took over last year.
Fighting was reportedly taking place in nearby villages between the jihadists and Kurdish militia group the People's Protection Unit (YPG) around the time of the attack.
"Sources in the area told Amnesty that, apart from the likely motive and the fact that ISIS operates there, they believe ISIS was responsible because of the clothing and behaviour of the perpetrators and the flag they were carrying," the rights group said, quoting sources in the area.
A hospital in the area confirmed it had received 15 bodies on the same day, with most victims receiving gunshot wounds to the head.
Officials from the YPG militia and its political affiliate, the Democratic Union Party (PYD), say they have lost more than 500 fighters repelling ISIS, al-Nusra and allied Islamist groups.