LONDON — A U.S. Army officer stationed at a NATO headquarters in Britain has been arrested on suspicion of spying for Russia, according to a U.S. official briefed on the case, in what would be one of the most serious alleged breaches of a Western military alliance in years.

The suspect is a senior officer based at Imjin Barracks in Gloucestershire, which houses the headquarters of NATO's Allied Rapid Reaction Corps — a multinational force built to respond quickly to crises across the alliance. British authorities have not publicly named the suspect, and officials have released few details about what evidence led to the arrest or what specific information investigators believe may have been compromised.

Neither U.S. nor U.K. officials have said how long the investigation had been underway before the arrest, and it remains unclear whether the officer has been charged. The arrest was first reported this week and has not been confirmed in detail by the Pentagon, which said only that it was aware of the matter and was cooperating with British authorities.

The case lands amid a broader run of Russia- and Iran-linked security scares rattling U.S. and British installations in recent days, including reports that Washington moved military aircraft off a second U.K. base over a separate threat and that federal prosecutors indicted three Iran-backed operatives accused of plotting an attack on a different British air base. Taken together, the incidents have put NATO's U.K.-based infrastructure under unusually intense scrutiny.

Espionage cases involving active-duty U.S. personnel stationed overseas are rare and typically trigger reviews of security clearances and access protocols across the implicated installation. A spokesperson for NATO's Allied Rapid Reaction Corps did not respond to requests for comment on whether operations at Imjin Barracks had been affected.