The Syrian Observatory for Human Rights claims 21 civilians have been killed in a US airstrike near Raqqa, Islamic State's stronghold in Syria.
The offensive to liberate the city from the grip of Isis began on Monday after coalition forces, backed US bombardments, had encircled the city for months.
However the UK-based monitoring group claims the group of civilians were caught up in the strike as they tried to flee the city and the upcoming battle.
The director of the organisation said women and children had been killed by the airstrike.
If Raqqa is liberated, it will be a major blow to the terror group, as it forms the de-facto capital of the so-called caliphate.
Isis took Raqqa in 2014 as it swept across Iraq and Syria, capturing vast swathes of territory and rocketing to global prominence almost overnight.
Just two weeks ago (May 26) Syrian state media outlets claimed US-led coalition strikes had killed at least 35 civilians. Earlier, in April, it was claimed that US-led strikes had killed more civilians than Isis or Russian-led forces over the previous month.