Al-Balad is the main airbase for Iraq's F-16s, which it bought from the US to upgrade its air capacities.
The base had held a small US Air Force contingent as well as American contractors, but a majority had been evacuated following tensions between the US and Iran over the past two weeks, military sources told AFP.
"About 90 percent of the US advisers, and employees of Sallyport and Lockheed Martin who are specialised in aircraft maintenance, have withdrawn to Taji and Erbil after threats," one of the sources said.
"There are no more than 15 US soldiers and a single plane at al-Balad," the source added.
Military bases hosting US troops have been subject to volleys of rocket and mortar attacks in recent months that have mostly wounded Iraqi forces, but also killed one American contractor last month.
That death set off a series of dramatic developments, with the US carrying out strikes against a pro-Iran paramilitary group in Iraq as well as a convoy carrying top Iranian and Iraqi commanders outside Baghdad airport.