The US said its "partnered forces" had requested an airstrike after identifying what they said was an IS "fighting position"
The American military admits a coalition airstrike has mistakenly killed 18 US-backed fighters in Syria.
Those who died south of the city of Tabqa 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, and coalition aircraft had been given wrong coordinates..
Previously, Russia strongly condemned the attack, saying it had violated international law and damaged relations between Moscow and Washington.
The Russian military vowed to help Syria strengthen its air defences following the airstrikes on an airbase outside of Homs.
This is a developing story...