A U.S. Air Force (USAF) F-15C Eagle fighter jet is pictured on the tarmac at RAF (Royal Air Force) Lakenheath, in eastern England, June 15, 2020. CHRIS RADBURN/AFP/Getty

The U.S. Air Force on Tuesday said that there had been more sightings of drones flying near bases used by U.S. forces in the United Kingdom, adding it had not yet identified them as hostile.

"We can confirm there were sightings yesterday (Monday) during nighttime hours and can only confirm that the number fluctuated, and varied between the bases over the night," a USAF in Europe spokesperson said in a statement sent to Agence France-Presse by the 100th Air Refueling Wing, which is based at the Royal Air Force base in Mildenhall, eastern England.

Mildenhall is one of three bases where drones have been spotted intermittently since November 20. The other two are the nearby RAF Lakenheath and RAF Feltwell.

The USAF said in a statement Monday that "small unmanned aerial systems were spotted in the vicinity of and over Royal Air Force Lakenheath, RAF Mildenhall and RAF Feltwell between Nov. 20 and Nov. 24."

The drones ranged in sizes and configurations, it added. In Tuesday's update, the USAF said there had been no impact to the bases' residents or infrastructure, and that the drones "have not been identified as hostile."

"However they are still continuously being monitored to ensure the safety and security of the installation," it added.

"I can confirm the situation is still ongoing and our units continue to monitor the airspace and are working with host-nation authorities and mission partners to ensure the safety of base personnel, facilities, and assets," a U.S. Air Forces in Europe spokesperson told CBS News on Tuesday.

A spokesman for the UK's Ministry of Defense said that it took "threats seriously" and was supporting the USAF response. CBS News' partner network BBC News said the U.K. military had deployed about 60 additional British troops to the bases to help aid in U.S. efforts to determine who is responsible for the drones.

The personnel have been sent to assist the US Air Force (USAF) in its investigation, the BBC understands.

While no entity or third nation has been named as a possible source of the unexplained drones, several major U.S. adversaries, including Russia and Iran, have been accused of increasing their espionage and sabotage activities in Europe this year.

U.S. fighter jets based at Lakenheath were part of the American effort to help shoot down missiles launched by Iran at Israel on April 13 — the first of two Iranian missile attacks on Israel this year.

Tension between Russia and the U.S. and Britain has also been extremely high amid the ongoing war in Ukraine, with British fighter jets scrambled only a week and a half ago to monitor a Russian reconnaissance aircraft that was flying close to U.K. airspace. That came just days after NATO jets were mobilized when Russian aircraft were spotted over the Baltic Sea and off the coast of Norway.

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