At least 56 civilians have been killed in air strikes conducted by the U.S.-led coalition around a stronghold of the Islamic State (IS) extremist group in northern Syria.
The Syrian Observatory for Human Rights, a London-based monitoring group, said the air strikes on July 19 hit the villages of Tokhar and Hoshariyeh north of the city of Manbij.
The predominantly Kurdish U.S.-backed Syria Democratic Forces (SDF) have been trying to capture Manbij from IS militants since the end of May.
Pentagon spokesman Adrian Rankine-Galloway said the United States was aware of the reported casualties and was conducting a "credibility assessment."
Amnesty International said the air strikes could represent the highest civilian casualties since the U.S.-led coalition against IS fighters began air operations in Syria in 2014.
"There must be a prompt, independent and transparent investigation to determine what happened, who was responsible, and how to avoid further needless loss of civilian life," said Magdalena Mughrabi, the international rights group's Middle East director.