The government is not confident that it knows how many people eligible to come to the UK remain in Afghanistan, Dominic Raab says.
Around 15,000 have been evacuated since the country fell to the Taliban.
Addressing MPs, the foreign secretary said he would be leaving for the region later for talks on those left behind.
He also said that the UK was “caught out” by the speed of the fall of Kabul, with intelligence expecting it to hold until the end of the year.
But the chairman of the Foreign Affairs Select Committee, Tom Tugendhat, said a Foreign Office risk report from 22 July predicted a “very real danger of cities collapsing” after US forces withdrew.
The Tory MP later told BBC Radio 5 Live that he was “a bit surprised” that Mr Raab did not seem to know about the document.
The Foreign Office said it was a standard monthly report which did not contain intelligence assessments.
Mr Raab was grilled by the committee’s MPs on a number of aspects of the UK’s withdrawal from Afghanistan last month.
Since the Taliban took control of the country in August, the foreign secretary has faced scrutiny over his handling of the situation.
There were calls for him to resign from opposition MPs after it emerged he had been unavailable to make a phone call about evacuating interpreters from Afghanistan while he was on holiday in Crete.
Mr Raab later said that “with hindsight” he would not have gone away, but dismissed accusations that he was “lounging on the beach” as “nonsense”.
The committee asked him for more specific dates for his holiday, but he refused to answer, saying the line of questioning was a “fishing expedition”.
The last UK plane evacuating people from Afghanistan took off on Saturday, with the US following on days later to stick to the 31 August deadline declared by US President Joe Biden.
Boris Johnson said the “overwhelming majority” of those eligible to come to the UK, such as UK nationals and Afghans working with the British, had been flown out.
But Mr Raab said he could still not give precise numbers.
Pushed by the committee on the number of UK nationals still in the country after the final flight, the foreign secretary said he believed it was in the “low hundreds”.
He also said the UK had flown out a number of people at risk, including 287 journalists, 65 women’s rights activists and nine judges.
But, while he stood by what the prime minister had said, Mr Raab was “not confident with precision to be able to give you a set number” of those who were eligible to come to the UK, but had not made it on to evacuation flights.
These groups include Afghan nationals and their families who worked with British forces during their time in the country and asylum seekers who could be eligible to come to the UK under international law.
Mr Raab also admitted that some of the people left behind included guards who had secured the British embassy in Kabul, after buses sent to pick them up and take them to the airport were not let through.
Questions won’t go away without concrete answers
Analysis by Ione Wells, BBC political correspondent
The foreign secretary appeared to bristle at some of the questions thrown at him by MPs.
MPs were frustrated by a lack of detail from Dominic Raab on key questions, like the specific number of people eligible for evacuation who had been left behind.
Mr Raab appeared equally irritated by questions on when exactly he went on holiday – refusing to provide any precise dates.
He had a heated exchange too with Labour’s Neil Coyle, who accused him of refusing to answer questions as they proceeded to speak over one another.
The MPs are unlikely to leave the session satisfied with the answers they got, with chair of the Committee, Tom Tugendhat, wrapping up by accusing Mr Raab of fire-fighting.
Certainly, as Parliament returns next week, questions for Mr Raab on key issues like how many people are left behind, how the government plans to get them out, and why ministers were taken by surprise by the speed of the Taliban’s takeover will not go away until concrete answers are provided.
He faced more questions about the embassy, including around reports from the Times that documents identifying Afghan workers and job applicants were found after the building was abandoned.
The foreign secretary said he had ordered a “full review” of the embassy’s closure as a result of the story, but said all the names that the newspaper passed on to the government were now safely in the UK.
Mr Raab was also pressed on reports that a portrait of the Queen had been left behind in the building, which the Taliban now had in its possession.
He told the Committee he had ordered anything linked to the government to be “destroyed” to stop an “attempted propaganda coup” by the Taliban.
And Mr Raab was also pushed over intelligence assessments in the run up to Kabul falling.
He revealed the information he had been given predicted a “steady deterioration” in the country, rather than the swift fall that happened, and that it was thought unlikely that Kabul would be taken by the Taliban before the end of the year.
But he said the UK had been making contingency plans since June to be able to react if the situation moved faster.
Asked by Tory MP Bob Seely whether the government had been “caught on the hop” because of an “intelligence failure”, Mr Raab replied: “We’ve got a very professional way of approaching these things but when they’re wrong… you need to look at how you correct that.”
But the foreign secretary seemed unaware of a risk report from his own department.
‘Fall of cities’
Mr Tugendhat read from the report dated 22 July that said: “Peace talks have stalled. The US/Nato withdrawal is resulting in rapid Taliban advances.
“This could lead to fall of cities, collapse of security forces, Taliban return to power, mass displacement and significant humanitarian need. The embassy may need to close if security deteriorates.”
Mr Raab asked: “I’m sorry, [what’s] the source of that?”
The chair replied: “It’s your principal risk report.”
An FCDO spokesperson said later after the hearing: “The principal risk register is a standard monthly report for the management board which does not contain intelligence assessments.
“It is an internal document which sets out potential risks to the organisation for planning purposes including around duty of care to staff.”
She added: “It is simply wrong and misleading to suggest this document is in any way at odds with our detailed assessments of the situation in Afghanistan or our public position throughout the crisis.
“The July document makes clear that our central planning assumption at the time was that the peace process in Afghanistan would run for up to a further six months.”
The foreign secretary told the MPs that the “central assessment remained until late that the deterioration would be incremental”, but the government had continued to make contingency plans for a “more rapid deterioration”.
Shadow foreign secretary Lisa Nandy called her government counterpart’s appearance “a staggeringly poor showing”, saying he was “out of his depth”.
She said Mr Raab was “unprepared for hard questions, unwilling to admit mistakes, unable to answer basic questions about how many British nationals have been left behind.
“Unsure who he had spoken to, about what or when. Unable to offer any advice to those still in danger in Afghanistan.”
Ms Nandy added: “Britain is now weaker in the world and faces greater threats to our national security. Nobody could watch today’s session and conclude we have a government capable of rising to the challenge.”