Iraqi military sources said a rocket launcher fixed on the back of a small truck had been parked in a rural area about 7 km (4 miles) to the east of the base, with at least two rockets ready to be fired toward Ain al-Asad.
The US airstrike destroyed the launcher, an army official said.
US-led coalition officials were not immediately available to comment on the strike.
The United States has 900 troops in Syria and 2,500 more in neighboring Iraq and says their mission is to advise and assist local forces trying to prevent a resurgence of Islamic State, which in 2014 seized large swathes of both countries but was later pushed back.
US military has been attacked 100 times since war began
Since the Israel-Hamas war began last October, the US military has come under attack at least 100 times in Iraq and Syria, usually with a mix of rockets and attack drones.
Iran-aligned militia groups in Iraq and Syria oppose Israel's campaign in Gaza and hold the US partly responsible.