British police declared a major incident near Royal Air Force (RAF) Fairford in Gloucestershire and began evacuating homes in the nearby village of Whelford after several men were arrested on suspicion of offenses under the Explosives Act.
The Army's explosive ordnance disposal team is examining several vehicles, Gloucestershire Constabulary said, adding that it believes the incident is contained.
RAF Fairford hosts U.S. B-1B bombers used in strikes in the U.S.-Iran war.
The base has reportedly raised its security posture to the highest level after specific threat intelligence.
Police have not said whether the arrests are linked to the base.
"A number of properties are currently being evacuated in the Whelford area following the declaration of a major incident," Gloucestershire Constabulary said in a statement.
"It follows the arrest of several men on suspicion of offenses under the Explosives Act," the force said.
"The Army's specialist explosive ordnance disposal team are currently examining a number of vehicles," it said, adding, "The local residents are being evacuated to a nearby leisure centre."
"A cordon is currently in place, but we'd like to reassure people that we believe this incident is contained," police said, noting, "We are working closely with our partners and are enacting the well-rehearsed plans we have in place."
The U.S. Department of War raised the force protection condition at RAF Fairford to FPCON Delta after receiving specific and credible intelligence about a threat to the installation, Faytuks Network reported.
The precise nature of the threat was not clear.
FPCON Delta is the most restrictive level of the Pentagon's force protection system. It applies in an immediate area where a terrorist attack has occurred or when an attack is considered imminent.
Additional military security vehicles arrived at Fairford on Thursday and Friday, and plane spotters reported increased police activity around the base. FPCON Charlie is reportedly in place at RAF Lakenheath and possibly RAF Mildenhall.
RAF Fairford is a major operating location for U.S. Air Forces in Europe. Aircraft movement records show B-1Bs flying repeated sorties from the base throughout September.
Satellite imagery shows the large B-1B contingent remains but is dispersed across the airfield, the outlet reported.
Aviation notices list the base as open for official business only, with a temporary transponder mandatory zone in effect through October. Those notices do not by themselves establish a link to the current threat.