Eighteen American-backed fighters have mistakenly been killed in an airstrike by the US-led coalition in Syria.
Those who died south of the city of Tabqa on Tuesday were members of the Syrian Democratic Forces (SDF), said the Pentagon.
The SDF is an alliance of Arab and Kurdish fighters who are battling Islamic State.
The US said its "partnered forces" had requested an airstrike after identifying what they said was an IS "fighting position".
But the target location was in fact where Syrian Democratic Forces were, rather than IS militants. The coalition aircraft had been given wrong coordinates, the US said.
The coalition has offered its "deepest condolences" to the members of the SDF and their families.
The SDF, with the help of air and ground support from the coalition, has surrounded the IS stronghold of Tabqa.
It is considered an important area ahead of the main offensive for Raqqa, which is the insurgents' de facto capital in Syria.
Several nations have lent their air power to the coalition to defeat the jihadist group.
But it is not clear which air force was behind the deadly strike.
Meanwhile, Syrian president Bashar al Assad says a deadly toxic gas attack widely blamed on his forces is a "100% fabrication" .
Speaking in his first interview since the incident, Mr Assad said he gave "no order to make any attack" and claimed "even if we had (chemical weapons) we wouldn't use them."
The Syrian leader said it was unclear who carried out the attack in the rebel-held town of Khan Sheikhoun on 4 April, which killed more than 80 people.