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An airstrike by US-led coalition accidently killed 18 allied fighters battling Islamic State [IS] militants in northern Syria this week, the US military said on Thursday.

The coalition aircraft were given wrong coordinates by partner forces, predominantly-Kurdish Syrian Democratic Forces [SDF], the US central command said.

The strike on Tuesday was intended to target Islamic State militants south of their Tabqa stronghold but hit SDF positions instead.

"The strike was requested by the partnered forces, who had identified the target location as an IS fighting position," a coalition statement read.

"The target location was actually a Syrian Democratic Forces fighting position."

SDF troops backed by US-led coalition air power and other military assistance have been fighting for control of Tabqa since last month. 

The town is considered an important waypoint ahead of the main offensive for Raqqa, the IS group's last bastion in Syria.

The US-led coalition offered its "deepest condolences" 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 IS despite this tragic incident," the statement said.

Officials were assessing the cause and would "implement appropriate safeguards to prevent similar incidents in the future," the statement added.

The SDF is a local Arab-Kurdish force that the US-led coalition is supporting with arms, airstrikes, training and advice as they fight IS.

Agencies contributed to this report