Two Latvian men have been arrested inside a military base in south-east England that hosts US and NATO intelligence centres after the perimeter fence was breached, police said.
The arrests come amid heightened security concerns at UK military bases following a separate incident last month.
Counter-terrorism police said on Thursday that the two men, aged 36 and 32, were detained at Royal Air Force base Molesworth in Cambridgeshire.
They are being held on suspicion of trespassing and entering a prohibited site "for purposes prejudicial to the United Kingdom", according to police.
A security patrol discovered an abandoned vehicle near the base late on Wednesday night. Police found a "small hole" in the perimeter fence before identifying and arresting the two men in the early hours of Thursday.
The men were taken to a Cambridgeshire police station and remain in custody. Both are Latvian nationals resident in the UK.
The arrests follow a serious incident at RAF Fairford in western England on 27 September, which led to the arrest of seven men, all in their 20s, who were later released on bail. One of them is a dual British-Iranian national.
No link to RAF Fairford
Given the nature of the incident, UK counter-terrorism police are leading the investigation into the security breach, police said.
"We are very conscious of the increased interest and concern over military sites recently," said Commander Helen Flanagan, head of London's Counter Terrorism Command.
"But at this stage, nothing suggests this incident is linked to the events at RAF Fairford," she added.
Flanagan said the investigation at RAF Molesworth is at an early stage but inquiries would focus on "any potential motive for the two men being at the site".
RAF Molesworth, formerly a second world war airfield, is now a non-flying facility operated by the US Air Force.
US intelligence operates its command centre for Europe and Africa at the base near Huntingdon. It is also a NATO intelligence centre, according to a CIA website.
On Sunday, US military aircraft left the RAF base at Fairford in Gloucestershire unexpectedly after three suspicious vehicles were seen near the base, prompting a serious incident to be declared.
That base has been used by the US military to carry out airstrikes during the war against Iran.
