The partnered forces believed that the SDF's position belonged to the Islamic State, the statement said. The dam, along with the surrounding town and adjacent airfield, will also provide Syrian and coalition forces with a prime "launching point" for future US and coalition-led operations once the looming advance on ISIS's self-styled capital of Raqqa begins, he added.
An air strike against the Islamic State (IS) group in northern Syria on Tuesday by the USA -led anti-terror coalition mistakenly killed 18 members of allied forces there, the Pentagon said Thursday.
The strike Tuesday by an aircraft from the USA -led coalition was called in by American allies, the US military says.
This story will be updated.
"The coalition has been working with the SDF for more than 2 years and have conducted thousands of strikes", the official said in an email.
Mansour said 17 SDF members had been killed in the strike south of Tabqa, a strategically important town south of Raqqa - one fewer than the number given by US Central Command.
A U.S. Air Force F-15E Strike Eagle fighter aircraft roars off into the sky over Afghanistan Oct. 6.
The alliance, which is supported by US-led coalition air strikes and special forces advisers, surrounded Tabqa in early April.
"The Coalition is assessing the cause of the incident and will implement appropriate safeguards to prevent similar incidents in the future", the Pentagon statement said.
The SDF is a local Arab-Kurdish force that the USA -led coalition is supporting with arms, air strikes, training and advice as they fight IS.
In response, coalition forces said they routinely strike ISIS targets in the area and would review reports of civilian casualties. It said that the airstrike showed that militants have chemical weapons in their possession.
The SDF is a USA -backed collective of moderate Kurdish and Syrian Arab rebel groups who have been trained and armed by the US military.
Activist groups and Syrian state media also allege that coalition forces carried out an airstrike on a school in al-Mansoura, west of Raqqa, where refugees were sheltering, killing dozens.
The U.S. Central Command issued a statement on Thursday acknowledging the strike.
The bombing marks the worst confirmed friendly-fire incident in the almost three-year-old war against the terrorist group.