A coalition air strike accidentally killed 18 members of a US-backed Arab-Kurdish alliance fighting the Islamic State group near a key town in northern Syria, the US-led coalition said Thursday.

Tuesday's strike occurred south of Tabqa, a strategically important town on the Euphrates River next to a dam and military airport, and close to the IS stronghold of Raqa.

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

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

SDF troops backed by 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 Raqa, the IS group's last bastion in Syria.

The 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 ISIS 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, air strikes, training and advice as they fight IS.