Two US personnel suffered minor injuries after a rocket attack on a base in eastern Syria, US officials said on Thursday.
The officials, speaking on the condition of anonymity, said one of the personnel had already been treated and released while the second was being checked for traumatic brain injury.
The officials said this was based on initial information and could change.
While it was not clear who had carried out the latest attack, Iranian-backed forces have targeted US forces in Iraq and Syria occasionally.
Syria's main oilfields are in areas controlled by US-backed forces and their proceeds are a major source of financing for the SDF forces.
Iranian-backed extremists are heavily concentrated west of the Euphrates River in Deir al-Zor province, where they get supplies from Iraq through the al-Bukamal border crossing.