At least 43 people were killed in air strikes by the US-led coalition on the pocket held by the Islamic State group in eastern part of the country in Syria.
The Syrian Observatory for Human Rights said that 17 children were among 36 Islamic State family members killed in Abu Husn village of Deir Ezzor province near the Iraq border.
The Britain based monitor said, another seven victims had not yet been identified as either civilians or IS fighters.
The US-led coalition has been backing a Kurdish-Arab alliance called the Syrian Democratic Forces (SDF) fighting to expel the militants from the pocket around Abu Husn.
Observatory chief Rami Abdel Rahman said, it is the highest death toll in coalition air strikes since the SDF launched its attack against the IS pocket in September.