A misdirected airstrike by the U.S.-led coalition earlier this week killed 18 allied fighters battling the Islamic State of Iraq and the Levant group (Isil) in northern Syria, the U.S. military said on Thursday.

The American Central Command said coalition aircraft were given the wrong co-ordinates by their partner forces, the predominantly-Kurdish Syrian Democratic Forces (SDF), for the strike on Tuesday that was intended to target IS militants south of their Tabqa stronghold, near the extremists' de facto capital, Raqqa. The strike hit an SDF position instead.

Several nations have lent their air power to the U.S.-led coalition to defeat the Islamist group, and it wasn't clear which air force was behind the errant strike.