Five men have been arrested and remain in custody, as counter-terror police lead an investigation at a major incident near RAF Fairford.
Gloucestershre Police say they received a call at about 00:45 BST relating to "three suspicious vehicles" travelling towards the base.
Residents in Whelford, a nearby village, have been evacuated and taken to a leisure centre while investigations continue.
The BBC's Duncan Kennedy is at the base, which the US Air Force have used to to launch attacks on Iran since March.
