A Britain-based war monitor says the death toll in Saturday's air strikes by the US-led coalition is the highest since the SDF launched its attack against the Daesh pocket.
Air strikes by the US-led coalition on Saturday killed 43 people, mostly civilians, in a holdout of the Daesh in eastern Syria, a Britain-based monitor said.
Seventeen children were among 36 Daesh family members killed in the village of Abu Husn in Deir Ezzor province near the Iraqi border, the Syrian Observatory for Human Rights said.
Another seven victims had not yet been identified as either civilians or Daesh fighters, it said.
The US-led coalition has been backing the Syrian Democratic Forces (SDF) fighting to expel militants from the pocket around Abu Husn.
The SDF is largely made up of the YPG - the Syrian affiliate of the PKK terror group.
"It's the highest death toll in coalition air strikes since the SDF launched its attack against the Daesh pocket" in September, Observatory chief Rami Abdel Rahman said.
The coalition has repeatedly said it does its utmost to prevent civilian casualties.
"The avoidance of civilian casualties is our highest priority when conducting strikes against legitimate military targets with precision munitions," spokesman Sean Ryan told AFP this week.
Daesh overran large swathes of Syria and neighbouring Iraq in 2014, proclaiming a "caliphate" in land it controlled.
But the militant group has since lost most of it to various offensives in both countries.
In Syria, the group has seen its presence reduced to parts of the vast Badia desert and the pocket in Deir Ezzor.
The SDF in September announced an assault to oust militants from the eastern pocket, which includes the town of Hajin and the village of Al-Shaafa.
The alliance made slow advances until last month when a tough fightback pushed the SDF out of the whole of the Daesh pocket.
Following the setback, hundreds of SDF fighters were deployed to the area's outskirts as reinforcements.
On Sunday, the SDF said it was resuming its offensive against Daesh after "intensive contacts" with the coalition and "strong diplomatic activity" to defuse the crisis.
Since 2014, the coalition has acknowledged direct responsibility for over 1,100 civilian deaths in Syria and Iraq, but rights groups put the number killed much higher.
Syria's war has killed more than 360,000 people since it erupted in 2011 with the brutal repression of anti-regime protests.