Armed Forces Minister James Hebby said an “imminent and fatal attack” could occur at Kabul airport within hours.
Hebe told Sky News that the “horrific truth” was that intelligence was “more certain than ever” of the possibility of an attack at the airport or processing centers used to assess refugees.
When asked if the attack could take place in the next few hours, he replied: “Yes.”
The defense minister explained that “the window of opportunity to evacuate people is closing,” but he did not say when the last flight would leave.
He added that the UK government was “doing all it can to keep those who have not left safely”.
“We are doing our best to look at other alternative provisions that we may be able to make, but we obviously won’t make it public as that would make alternative approaches an end in themselves.”
It comes as a senior British source told Sky News that there is a “very high risk of a terrorist attack” against the Kabul evacuation that involved the UK, US and other allied forces.
The most worrisome group is the Islamic State group Afghanistan The source said that the so-called ISIS-Khorasan or ISIS-Khorasan.
The threat puts additional pressure on the operation to help people flee the nation that the Taliban has taken over.
“Intelligence over the course of the week has become more and more certain about the imminent, deadly and highly credible attack on the airport or handling centers used by Western forces,” Hebei told Sky News.
“In all conscience, with the clock still ticking towards the end of the month of course, we had to share this very real threat with the people of Kabul and advise them to stay away from the airport, not to come to the airport.
“And I realize this is causing real desperation for a lot of people who are not yet out of the country, but we are not being overly vigilant, the threat – while I can’t give you the details – is very credible and very deadly.”
Asked how many flights are expected to take off from Kabul, Mr. Hebei added: “Well, we still have 11 scheduled flights to depart in the next 24 hours.”
He said the intent was to “continue treating people” on Thursday, but “in fact, we have to share the advice we’ve given” to people to avoid gathering at the airport.
Mr Hebe continued: “This means we have to be very clear with people about the risks and do everything we can to mitigate them, but we don’t have the number of troops on the ground that we need to be able to go in the city, find the threat and remove it.”
“This therefore means that we have to deal with the threat as it is and get as many people out as possible in the remaining time.”
Pressed again when the last evacuation flight was scheduled to leave Kabul airport, the defense minister replied: “Our judgment is that the evacuation flights should not be declared over.”
“We have to take responsibility for how we deliver the rest of our plan,” he added.
“Once the military withdrawal is fully done, we need not make any comment on that – because as you can imagine, with each flight leaving, there are fewer forces left to secure the ground there afterward, so we are more vulnerable as we build up to that point.
“So, I’m afraid the Department of Defense should be accountable to our forces and those on the outside in not sharing every detail and I apologize for that.”
Elsewhere on his Thursday morning radio tour, Mr Hebe confirmed that 1,988 people had left Kabul over eight RAF flights in the past 24 hours.