British police said they had arrested several men on suspicion of preparing bomb plots, Reuters reported.
Military officers inspect several vehicles after security at the British air base "Fairford", used by the US air force, was tightened.
The British government has authorized the US to use the base in southwest England to strike Iranian missile sites linked to attacks on ships in the Strait of Hormuz.
Armed police blocked access to the main entrance to the base. Around 30 emergency services vehicles, including police cars, ambulances and fire engines, are stationed in the nearby village of Fairford.
"A number of properties in the Welford area are currently being evacuated after a serious incident was reported. This follows the arrest of several men on suspicion of breaches of the Explosives Act," Gloucestershire Police said. Welford is located near the air base.
A British Army bomb disposal team is examining several vehicles.
A US Air Force spokesman said he would not comment on specific measures to protect the base, adding that the situation would continue to be closely monitored.
He said the US Air Force was constantly assessing various factors to determine what measures to protect military facilities and personnel needed to be put in place or changed.
In July, Britain said its armed forces were ready to defend the country in the event of an attack. The statement followed a warning from Iran's Islamic Revolutionary Guard Corps (IRGC) that Britain should not allow US bombers to take off from "Fairford".
British ministers have also briefed critical infrastructure operators on threats linked to Russia. The government has expressed growing concern about possible sabotage attempts and cyberattacks, while Moscow denies carrying out hybrid attacks against Western countries.