British Armed Forces Minister James Heappey told the BBC on Thursday there was ”very, very credible reporting of an imminent attack” at the airport, possibly within “hours.”

Heappey conceded that people are desperate to leave and “there is an appetite by many in the queue to take their chances, but the reporting of this threat is very credible indeed and there is a real imminence to it.”

“There is every chance that as further reporting comes in, we may be able to change the advice again and process people anew, but there’s no guarantee of that,” he added.

Late Wednesday, the U.S. Embassy warned citizens at three airport gates to leave immediately due to an unspecified security threat. Australia, Britain and New Zealand also advised their citizens Thursday not to go to the airport, with Australia’s foreign minister saying there was a “very high threat of a terrorist attack.”