Coalition aircraft were given the wrong coordinates for a strike on 11 April, US Central Command said on Thursday, resulting in a strike on an SDF position and the deaths of 18 partnered fighters in the north of the country.
The inaccurate coordinates reportedly came from the SDF itself.
In the last month, a predominately Kurdish alliance of several fighting groups has managed to surround the Isis stronghold of Tabqa with both air and ground support from the US-led coalition.
It is not yet clear which air force of the several nations which lend air power to defeating Isis was behind the strike.
“The coalition’s deepest condolences go out to the members of the SDF and their families. The coalition is in close contact with our SDF partners who have expressed a strong desire to remain focused on the fight against Isis despite this tragic incident,” a statement from US Central Command said.
“The coalition is assessing the cause of the incident and will implement appropriate safeguards to prevent similar incidents in the future.”
In its own statement, the SDF described the incident as a “painful accident” which was the result of a “mistake”.
The alliance’s general command is coordinating with the US-led coalition’s investigation.
The incident is the latest in several recent US-led strikes against Isis and al-Qaeda in Syria and Iraq which have accidentally hit allied or civilian targets.
In the Iraqi city of Mosul last month, a US strike is believed to have killed up to 200 civilians in a single bombing.
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