The UK has now evacuated the "overwhelming majority" of eligible people from Afghanistan, Boris Johnson has said.
But the prime minister admitted the time remaining for evacuations may be "quite short", amid a serious terror threat from Islamic State Khorasan province (ISIS-K).
He added that around 15,000 people had already been evacuated by British troops.
It's been reported that as many as 1,500 eligible people could be left behind in the Taliban-ruled Afghanistan, but Armed forces minister James Heappey told ITV News he hopes it will be less than that.
He said there will be 11 more flights out of Kabul on Thursday, with capacity to extract 3,000 people, but declined to say whether there will be more on Friday, citing the security of troops.
The minister told ITV News the "imminent" terror threat from ISIS-K "would be highly lethal" and the "worsening security situation" in Afghanistan's capital means British forces will "probably get out fewer people" than had been hoped.
On the threat at Kabul airport, the PM said: "I think we have to be transparent about the risks, that we have to be realistic about what's going on, and you'll appreciate that there are Islamic State Khorasan province (Isis-K) terrorists out there.
"I can't go into the details, clearly. But we have to be mindful of the security of our personnel, but also of the Afghan people who are trying to get out."
Mr Johnson admitted that while the "lion's share" of eligible people had now been removed from the country, he recognised "there will be people who still need help".
The PM said the UK will "do everything we can to get everybody else" but admitted some people may need to flee the country via its land borders, with facilities in neighbouring countries ready to process them for resettlement in the UK.
Amid concerns the Taliban could block citizens from leaving, Mr Johnson warned the group it must allow people to leave if Afghanistan is to benefit from engagement from the wider world.
He said the G7 agreed that "safe passage for those who want to come out is the key precondition" of development aid as well as diplomatic and political relationships.
Mr Heappey said it is "not impossible" that the terror threat could reduce and the processing of people for evacuations could resume, "but that's not a guarantee that we can offer anybody".
He said the result of a terror attack on Kabul airport would be the "most gut-wrenching, miserable, abhorrent scenes that you could ever possibly imagine".