The UK’s rescue mission in Kabul reached its end on Saturday with hundreds of people feared to be left behind.
Gen Sir Nick Carter, head of the British Armed Forces, said it was heartbreaking that not everyone could be flown out by the August 31 deadline.
The civilian evacuation operation is ending on Saturday so the military can complete its withdrawal in what Gen Carter said would be a difficult final stretch.
Nearly 15,000 people have been airlifted out since the Taliban took over Kabul, including British citizens and Afghans who worked for Nato forces.
Remaining UK citizens were urged to avoid the airport owing to security fears that culminated in the terrorist attack on Thursday. Processing centres at the Baron Hotel have been closed.
Gen Carter said the chaos in Kabul had contributed to some people being stuck. He estimated the number left behind as being in the “high hundreds”.
“We are forever receiving messages and texts from our Afghan friends that are very distressing. We’re all living this in a very painful way,” he told the BBC’s Today programme.
“It’s gone as well as it could do in the circumstances. But we haven’t been able to bring everybody out, and that has been heartbreaking.”
As the civilian evacuation ends, Britain has begun pulling out its final contingent of troops and diplomatic personnel. Some landed at RAF Brize Norton in the early hours of Saturday.
Clearance was given for animal charity founder Paul Farthing to leave with his cats and dogs on a chartered plane, in a case that has caused controversy.
The military evacuation operation is taking place in the shadow of suicide bombings that killed dozens of people on Thursday, including British citizens.
Gen Carter said US troops would be “very challenged” in the coming days as the terrorist threat remained and Afghans continued to crowd the airport.
“The next three or four days are going to be really difficult on the ground and I think people need to reflect on that a bit,” he said.
Britain hopes to persuade the Taliban to allow people to leave by commercial means after August 31.
Prime Minister Boris Johnson hopes to use frozen Afghan funds and the prospect of international co-operation as leverage over the Taliban regime.
He spoke to German Chancellor Angela Merkel on Saturday and emphasised "any recognition and engagement with the Taliban must be conditional on them allowing safe passage for those who want to leave the country and respecting human rights", his office said.
The two leaders agreed on the need to prevent a humanitarian crisis in Afghanistan in the coming weeks, Downing Street said.
Resettlement offer stays open
Sir Laurie Bristow, the UK ambassador to Afghanistan, said on Saturday that the UK would do everything it could to help people left behind.
"It's time to close this phase of the operation now. But we haven't forgotten the people who still need to leave," he said. "Nor have we forgotten the brave, decent people of Afghanistan."
In a letter to MPs, ministers offered reassurances that the resettlement initiative for locally employed Afghan staff does not expire on August 31.
Another 5,000 people will be taken in over the next year under a separate scheme for Afghans at risk, such as women and girls.
MPs have described a huge workload as they try to help constituents with relatives in Afghanistan. The UK’s home, foreign and defence secretaries said in the letter that MPs should keep directing people to government helplines.
“We know that every single one of you will have been contacted about heart-breaking stories of those left behind in unimaginable circumstances,” they said.
“We are clear that the Taliban must ensure safe passage for these people out of Afghanistan and any engagement with them will emphasise this first and foremost.”