A senior soldier in the US Army has been arrested by British counter-terrorism police.
The 41-year-old man was arrested near Gloucester on Tuesday, on suspicion of offences under the National Security Act, the Metropolitan Police said.
The force said the allegations related to Russia. The FBI said the arrest involves allegations of espionage.
As part of the investigation, officers are carrying out searches at two locations in Gloucestershire.
Counter-terrorism police in the UK is working closely with the US military and the FBI.
FBI director Kash Patel said in a statement: "This important arrest involves allegations of espionage, which is of the utmost concern to the United States and our allies."
Deputy Assistant Commissioner Vicki Evans, from Counter Terrorism Policing, described the arrest as "significant".
"The allegations are serious and our investigation is very much ongoing," she said.
Brig Gen Sean Stinchon, commanding general of US Army Counterintelligence Command, said: "The alleged conduct of a senior solider, entrusted with sensitive information, is a sobering reminder that insider threats are real and persistent.
"I encourage anyone who thinks they've witnessed something suspicious to report it to their local counterintelligence office."
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