At least 18 members of the American-backed Syrian Democratic Forces (SDF) were killed in a US-led coalition air strike that mistakenly targeted them in Syria's Raqqa province.

In a statement released on Thursday, US Central Command (CENTCOM) said 18 SDF fighters died in the air raid south of the city of Tabqa on April 11. The attack was believed to be hitting members of Islamic State of Iraq and the Levant (ISIL, also known as ISIS).

SDF was founded in Syria's mainly Kurdish northeastern region in October 2015, and is made up of at least 15 armed factions, mostly fighters from the Kurdish People's Protection Units and the Free Syrian Army.

"The strike was requested by the partnered forces, who had identified the target location as an ISIS fighting position. The target location was actually a forward Syrian Democratic Forces fighting position," CENTCOM said.

"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."

The coalition added it is assessing the cause of the friendly fire attack.

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The UK-based Syrian Observatory for Human Rights said on Thursday at least 25 other SDF fighters were killed in clashes against ISIL in the suburbs of Tabqa.

The incident occurred as US-backed Syrian forces prepared to retake Raqqa, ISIL's stronghold in Syria, as they moved in from the city's north.

SDF captured the strategic Tabqa airbase from ISIL last month.The airbase is 45km west of Raqqa,

Raqqa province was taken by ISIL in January 2014 from the former al-Qaeda affiliate in Syria, and ISIL seized Tabqa's airbase from the Syrian government in August 2014.