A US citizen fighting for the Islamic State (IS) militant group in Syria has surrendered to a US-backed alliance, a defense official said Thursday.

"We are aware that a US citizen surrendered to the SDF on or about September 12," the official told AFP on condition of anonymity.

The SDF, or Syrian Democratic Forces, are a largely Kurdish alliance that also includes Syrian Arab fighters.

The group is supported by US weapons, air strikes and intelligence as they battle IS in Syria, where the fight is currently focused on Raqa and the Euphrates River valley.

The official did not provide additional details.

Earlier, Colonel Ryan Dillon, a spokesman for the US-led anti-IS coalition, said the SDF "has taken note" to ensure that any fighters they capture are brought to "the proper authorities."

In June, a US man who had traveled to Raqa in December 2015 was convicted of helping IS.

Mohamad Jamal Khweis had agreed to become a suicide bomber but surrendered to Kurdish peshmerga forces in northern Iraq after becoming disillusioned.