London’s Metropolitan Police on Sunday said five men had been arrested on suspicion of preparing a “terrorist act”, after an incident near an air base in southwestern England used by the United States Air Force.

“Counter Terrorism Policing can now confirm they are leading an investigation into an incident which took place overnight near RAF Fairford in Gloucestershire,” the Met said in a statement.

The police earlier said they had arrested several men on suspicion of offences under the Explosives Act and the army was examining a number of vehicles, after security was stepped up at the airbase in southwest England.

The British government gave authorisation to the US to use RAF Fairford to carry out strikes on Iranian missile sites that are attacking ships in the Strait of Hormuz.

A Reuters eyewitness said armed police were blocking access to the main gate of the base. Around 30 emergency vehicles, including police cars, ambulances and fire engines, were stationed in the village of Fairford near the base.

“A number of properties are currently being evacuated in the Whelford area following the declaration of a major incident. It follows the arrest of several men on suspicion of offences under the Explosives Act,” Gloucestershire police said in a statement, referring to a village near the airbase.

“The Army’s specialist explosive ordnance disposal team are currently examining a number of vehicles.”

A spokesperson for the US Air Force said they would not discuss specific force protection measures.

“The 501st Combat Support Wing and our fellow UK-based Wings remain vigilant and will take appropriate actions to ensure the safety and security of our US service members, civilians, contractors, and their families,” the spokesperson said.

“We constantly assess a variety of factors that determine what measures we institute or modify to protect our installations, our people and their families.”

Prime Minister Andy Burnham was being kept updated on the incident, a government spokesperson said.

“We are grateful to the emergency services, who have acted quickly to detain several suspects and put measures in place to keep the community safe,” the spokesperson said.

“It would be inappropriate to comment further whilst investigations are ongoing.”

Defence Minister Wes Streeting thanked “emergency and specialist teams” deployed to the scene.

“We can be reassured that this is being dealt with,” he said on X.

Fairford is the UK base of the 501st Combat Support Wing, a US air force administrative support wing.

Personnel there would “remain vigilant and will take appropriate actions to ensure the safety and security of our US service members, civilians, contractors, and their families” a statement said.

The MP for the area, Roz Savage, described the situation as “worrying”.

“I have contacted the police for further updates and I’ve been told that it is believed the incident is contained,” she said on X.

Britain said in July its armed forces were ready to protect the country from any attack after Iran’s Revolutionary Guards issued a warning not to allow US bombers to fly from Fairford.

British ministers have also briefed providers of critical infrastructure of the threat posed by Russia, with the government increasingly concerned about possible sabotage operations and cyberattacks. Russia denies mounting hybrid attacks on Western nations.