At least 10 US Air Force refuelling aircraft left a second airbase in Britain on Monday.
Flight tracking data seen by BBC Verify shows the aircraft left RAF Mildenhall in Suffolk and landed at other American bases in Europe and also at Prestwick Airport in Ayrshire.
The Wall Street Journal reported that this was in response to a threat from Iran but the BBC has not independently verified that claim.
Monday's departures come weeks after the US withdrew its bombers from RAF Fairford airbase over a suspected Iran-linked security incident.
Asked about Monday's flights, an MoD spokesperson told the BBC: “We do not routinely comment on the activity of other nations. Aircraft and other assets move regularly according to operational requirements.”
The BBC has contacted the Pentagon and the United States European Command (EUCOM) for comment.
Using public tracking site Flightradar24 BBC Verify has been able to see USAF KC-135 tankers leaving Mildenhall throughout Monday.
Two aircraft that left initially flew to US bases in Germany and Italy. The first KC-135 then returned to Mildenhall before heading east and landing at Spangdahlem airbase in Germany. The other aircraft - having flown circuits over Aviano airbase in Italy - flew on to and landed at Spangdahlem.
Flight-tracking data also shows eight other US KC-135s left Mildenhall later on Monday.
The aircraft that landed at Prestwick on Monday is with the Nebraska Air National Guard in the US so that could have been a regular stop before heading back to base.
Asked about the US evacuating aircraft from RAF Mildenhall, Education Secretary Lucy Powell told BBC Breakfast she could not comment on ongoing operational matters.
She added that "we take the security and safety of our bases incredibly seriously" and “we will always act on security intelligence when we need to.”
A US Air Force spokesperson said its aircraft "rotate between installations and operating locations for a variety of reasons including training and real world requirements".
"US military aircraft are hosted in accordance with access, basing, and overflight agreements with our allies and partners. Due to operations security, we will not provide further information on movement of assets," the spokesperson added.
Former UK National Security Adviser Lord Ricketts told BBC Radio 4's Today programme: “We are talking about an airbase which houses these large tanker aircraft. Of course, they are large targets, an airbase has a long perimeter, so they are inevitably vulnerable.”
He added: “Nobody knows at this stage what this information was. I suspect part of it is an abundance of caution.”
Speaking before reports emerged of the flights leaving RAF Mildenhall, US Secretary of State Marco Rubio was asked on Monday about Iranian threats against American diplomats overseas.
"We've addressed it by moving people away from the threat," he said, adding: “Obviously, we're always worried about the safety and security of our personnel and their families overseas. And so, when we see anything that could pose a potential threat beyond a certain threshold, we're going to address it.”
Monday's departures from Mildenhall follow a security incident at RAF Fairford, a US-operated airbase in Gloucestershire.
A major incident was declared after three suspicious vans were seen near that base in September. It later emerged that the US had pulled its bombers out of the base and returned them to America.
Two British nationals appeared in court in October in connection with an alleged Iran-linked plot. Eight men have been arrested as part of the counter-terror police investigation.
Source: BBC
--Agencies
