British police say they are responding to a "significant" incident at a Royal Air Force base used by the U.S. Air Force.
Police say it happened at RAF Mildenhall in Suffolk and urged the public to stay away from the area for the time being. Police said Monday that further details would be released shortly.
British media suggested an individual in a vehicle tried to force his way through a checkpoint, but this wasn't confirmed.
RAF officials referred calls to police.