A 41-year-old member of the US Army was arrested near Gloucester on Tuesday in England, the Metropolitan Police said in a statement.
Authorities were searched two locations in the Gloucestershire region, police said, adding: "The country to which the allegations pertain is Russia."
US authorities, the US Military and the FBI were closely involved in the investigation, police said.
Deputy Assistant Commissioner Vicki Evans, Senior National Coordinator for Counter Terrorism Policing called it a "significant arrest" as a result of "close collaboration between UK and US authorities."
"The allegations are serious and our investigation is very much ongoing," Evans said.
This is a developing story. More to follow...
