Police are responding to reports of a “significant incident” at RAF Mildenhall.
The US Air Force base in Suffolk is reportedly on lockdown amid reports of a car trying to ram the gates.
A brief statement on the base’s Facebook page said: “RAF Mildenhall locked down at 1pm today following reports of a disturbance on base.
“The base was locked down and emergency personnel are responding to the situation.
“Additional details will be provided as they become available.”
Two businesses on the base confirmed they were no longer on lockdown after the security alert.
A member of staff at Jumppin Jacks Funhouse in Mildenhall told the Press Association: “All the American children playing here suddenly left, but we didn’t hear anything or know why.”
RAF Mildenhall has previously been a potential target for a terror attack.
In May 2016 Islamic extremist Junead Khan was given a life sentence for planning a strike against American military personnel in Britain.
Khan, of Luton, used his job as a delivery driver to scout bases including Mildenhall and nearby RAF Lakenheath the previous year.
The trial of the “committed supporter” of Islamic State (IS) heard he plotted to fake a car accident before attacking personnel with a “Jihadi John”-style knife.
He was jailed for a minimum of 12 years after being convicted of engaging in conduct in preparation of terrorist acts.