The "massacre" targeted Manbej, a town controlled by the Islamic State militant group, in the northern countryside of Aleppo province, Xinhua news agency reported citing the Syrian Observatory for Human Rights.
The US-led anti-terror coalition has been backing the rebel's Syrian Democratic Forces to capture the town since May last year.
Capturing Manbej has a strategic importance to the US-backed rebels due to its location and depriving the IS from a key stronghold near Turkey.
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