The government has warned that those behind recent drone incursions over US airbases will be "shown the full force of the law".
Defence Minister Maria Eagle said it was "taking steps" after the mystery unmanned aerial vehicles (UAVs) were spotted over and around the three locations in Suffolk and Norfolk.
Sixty RAF personnel have been deployed to assist the US Air Force (USAF) in its investigation.
It has also emerged a fourth US airbase has been targeted.
"Protection of our personnel and bases is of course our highest priority," said Eagle, speaking during a debate on UK air and missile defences at Westminster Hall.
"For security reasons I won't go into specifics. But we are taking steps and we are aware of what's going on and doing our best to deal with it".
There have been multiple sightings of UAVs over RAF Lakenheath and RAF Mildenhall in Suffolk - between Saturday and Monday - and over RAF Feltwell in neighbouring Norfolk.
The USAF said the drones were not identified as "hostile".
RAF Fairford in Gloucestershire had also experienced drone incursions, a spokesperson added.
Four B-52 Stratofortress bombers landed at RAF Fairford earlier this month.
MPs were told the Civil Aviation Authority had flight restriction zones imposed around these bases.
"We will be making sure that anybody we manage to catch for engaging in this behaviour will be shown the full force of the law for their illegal activities," Ms Eagle continued.
A spokesman for the US Air Forces in Europe said it was monitoring the airspace and added: "We retain the right to protect our installations."
A Ministry of Defence spokesperson said on Tuesday: "We take threats seriously and maintain robust measures at defence sites."
