The news follows a number of reports in the past week of unmanned aerial vehicles spotted in the vicinity of RAF Lakenheath and RAF Mildenhall, both in Suffolk, and RAF Feltwell in neighbouring Norfolk.
Further drone activity had been reported overnight. The USAF said the vehicles were small in number and size.
A US Air Forces in Europe spokesman said: "We can confirm there were sightings yesterday during night-time hours and can only confirm that the number fluctuated and varied between the bases over the night."
The spokesman added that since the first sightings on 20 November there had been "no impact to residents or infrastructure and they have not been identified as hostile".
However, the drones are still being monitored "to ensure the safety and security of the installations".
RAF Mildenhall is primarily home to the USAF's 100th Air Refuelling Wing; RAF Lakenheath is home to USAF F-35A and F-15E fighter jets; and RAF Feltwell is mainly concerned with logistics and provides housing for military personnel.
A spokesman for the MoD, which owns the bases, said: "We take threats seriously and maintain robust measures at defence sites.
"We are supporting the US Air Force response."
The USAF has not said who it believed to be behind the incidents.