Aircraft left RAF Mildenhall in Suffolk and landed at other American bases in Europe and also at Prestwick Airport in Ayrshire
An MP said the government has confirmed there was a threat to RAF Mildenhall in Suffolk but not to the town itself.
More than 12 US Air Force aircraft left RAF Mildenhall weeks after the US withdrew its bombers from RAF Fairford over a suspected Iran-linked security incident.
Nick Timothy, the West Suffolk Conservative MP, said: "I spoke to a defence minister again last night and she confirmed to me that a threat was to the base and not to the town's civilians or civilian infrastructure.
"She also told me the threat was not imminent and the action that was taken was precautionary."
RAF Mildenhall is home to the 100th Air Refuelling Wing, the only permanent US air refuelling wing in Europe.
About 3,000 military personnel are stationed at the base, which is about 80 miles (130km) north of London.
Military personnel were pictured loading a US Air Force plane at RAF Mildenhall on Tuesday afternoon
Timothy said: "Information is still being kept very tight and obviously it's about national security, so there are limits to what the Ministry of Defence and US Air Force are likely to say publicly."
He encouraged people to be vigilant and report anything that might be out of the ordinary to the authorities.
"These are national security issues, and with these kinds of threats it's not like in a James Bond movie – you get fragments of information and you can't act with total certainty all the time.
"There's a reason why information is kept quite tight in these circumstances, but what I do always say and I sadly say it right now, is that subject to those obvious national security constraints, it's really important that the Ministry of Defence and the US Air Force communicate as openly as possible with residents so that people can stay informed and they can know what to look out for and what to do if they do see things out of the ordinary.
"That's why ministers have been talking to me over the last day or two," he added.
Monday's departures from RAF Mildenhall followed a security incident at RAF Fairford, a US-operated airbase in Gloucestershire.
The Ministry of Defence has been contacted for comment.
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