A misdirected airstrike has killed 18 allied fighters battling the Islamic State group in northern Syria, the US military says.
US Central Command said that coalition aircraft were given the wrong coordinates by their partner forces, the predominantly-Kurdish Syrian Democratic Forces, for a strike intended to target Islamic State militants south of their Tabqa stronghold.
The strike hit an SDF position instead, killing 18 fighters.
Central Command said the incident occurred on Tuesday.
The SDF, with the help of air and ground support from the US-led coalition, has surrounded Tabqa.
Several nations have lent their air power to the coalition to defeat the Islamic State group.
It is not clear which air force was behind the strike.
Earlier, Russia's foreign minister says Moscow expects the UN's chemical weapons watchdog to conduct an extensive probe into last week's chemical attack in Syria.
Sergey Lavrov said that inspectors from the Organisation for the Prohibition of Chemical Weapons should both visit the Syrian air base, which the US said had served as a platform for the attack, and the town of Khan Sheikhoun that was hit to get a full and objective picture.
He said Russia vetoed a Western draft UN resolution on Wednesday because it failed to mention the need to inspect the area of the attack.
The US blamed the Syrian government for launching the attack, but Russia claimed that the victims were killed by toxic agents released from a rebel chemical arsenal hit by Syrian planes.