British forces are scrambling to evacuate British citizens still in the country and native allies who worked with the UK during its occupation of Afghanistan, but many of those based outside the capital Kabul will be stranded, Ben Wallace has said.
"I've been really frank," the former army captain told ITV News, "there are still people who will not be able to get to Kabul and will just not be able to get out at this time."
Boris Johnson has called an emergency Cobra meeting for Monday afternoon to discuss the unfolding crisis an has recalled Parliament for a debate on Wednesday.
Defence Secretary Wallace said around 2,000 Afghans allied with British Army during the war, many as interpreters, have been given a "ticket to ride" to the UK on military planes "but that is not withstanding whether the Taliban will allow all people out of the country".
The group has apparently given assurances to the UK government via a Middle East country that the evacuation effort will be able to continue but "there are some voices on the right who are saying different things".
"Fundamentally we will do everything we can to help [people outside the capital] get to the airport."
He said the government is open to processing Afghans in third countries and will keep the process for evacuating them open "for years if necessary".
The government has also agreed that Afghans allied with the UK will be allowed to enter the country without a passport, so long as they have passed a vetting process.
But there's a "difficult balance" to strike when considering which Afghans should be allowed into the UK because "some of them haven't worked for us for 10, 12, 13 years".
"Some of them may have - and I'm afraid a very small percentage - some of them have links to the Taliban and we just can't bring them into the country - that would be totally wrong to do so."
Mr Wallace said the barrier to helping more people leave the country was how quickly they could be processed, rather than any issues with available aircraft.
Unsettling videos emerged from people desperately attempting to board planes leaving the country, with dozens scrambling into jam-packed doorways and others even holding onto aircraft as they took off in a bid to escape.
But Mr Wallace insisted that the airport is functioning, with 600 British troops having secured an area where planes have been able to leave and arrive.
He said 300 people were evacuated on Sunday night and the government was aiming to fly out a further 1,500 people over the next 24 to 36 hours or slightly longer.
The Taliban's takeover of Afghanistan appeared complete when fighters were pictured inside the presidential palace abandoned by President Ashraf Ghani, who fled the country while his forces gave up the city without a fight.
Labour leader Sir Keir Starmer has called on the government to "step up to the plate" by outlining to the international community what should be its priority, at a United Nations Security Council meeting this afternoon.
Concerns for Afghan women and girls were laid bare by the country's representative at the meeting, who said he is speaking for the millions "who are about to lose their freedom to go to school, to work, and to participate in the political, economic, and social life of the country".
Ghulam M Isaczai said: "We're extremely concerned about Taliban's not honouring the promises and commitments made in their statements at Doha, and other international fora.
"We've witnessed time and again how Taliban have broken their promises and commitments in the past. We have seen gruesome images of Taliban mass executions of military personnel and target killings of civilians in Kandahar and other big cities.
"We cannot allow this to happen in Kabul, which has been the last refuge for many people escaping violence and Taliban's revenge attacks."
Mr Isaczai called on the UN to put pressure on the Taliban "to prevent further violence, prevent Afghanistan descending into a civil war and becoming a pariah state".
He added: "Kabul residents are living in absolute fear right now."
Aside from the evacuation effort, Sir Keir said the focus should be "an assertion of the human rights of everybody in Afghanistan... particularly women and girls, and an agreement about safe and legal routes for refugees, because it is inevitable there is going to be a refugee crisis coming out of this".
"What I say to the government is, step up to the plate, show that urgency, and take these priorities into the meeting this afternoon," he added.
Following a meeting of the Government’s Cobra emergencies committee on Sunday, Prime Minister Johnson said his priority was to get UK nationals and Afghans who had worked with them out of the country “as fast as we can”.
Around 4,000 British nationals and eligible Afghans are thought to be in the city and in need of evacuation.
In a sign of the desperate situation, the British ambassador Sir Laurie Bristow was said to be helping the small team of diplomats still in the country to process the applications of those hoping to leave.
There was particular concern for the safety of Afghans who worked with British forces when they were in the country as interpreters and in other roles amid fears of reprisals if they fall into the hands of the insurgents.
There were desperate scenes at Kabul's international airports as people tried to escape Afghanistan after the Taliban seized control of the presidential palace
The Taliban insisted that they were seeking a peaceful takeover of power and were prepared to offer an amnesty to those who had worked with the Afghan government or with foreign governments.
However those assurances were being treated with deep scepticism by many British MPs amid reports of threats to those who remain and their families.
Labour called on the government to urgently expand the resettlement scheme for Afghans to ensure that none were left behind.
Sir Keir said: "We need to get UK nationals out, but we also have an obligation to all of those Afghans who helped and assisted the UK, and we shouldn't have nice distinctions between this type of person, this type of help, and that type of help.
"If those in Afghan have helped us, the UK, in our work in Afghanistan, we have got an obligation to them."