US forces are in the final phase of leaving Kabul, ending two decades of involvement in Afghanistan, and just over 1,000 civilians at the airport remain to be flown out before troops withdraw, a Western security official said on Sunday.

The country's new Taliban rulers are prepared to take control of the airport, said an official from the group that has swept across Afghanistan, crushing the US-backed government.

The Western security official, who asked not to be identified, told Reuters a date and time for the end of the operation was yet to be decided.

President Joe Biden has said he will stick by his deadline to withdraw all US troops from Afghanistan by Tuesday, 20 years after they invaded Kabul and ousted the Taliban government for shielding the perpetrators of the September 11, 2001 attacks.

"We want to ensure that every foreign civilian and those who are at risk are evacuated today. Forces will start flying out once this process is over," said the official, who is stationed at the airport.

The Western-backed government and Afghan army melted away as the Taliban entered the capital on August 15, leaving an administrative vacuum that has bolstered fears of a financial collapse and widespread hunger.

Under a deal with the United States, the Taliban have said they will allow foreigners and Afghans who wish to leave to fly out. The United States and its allies have taken about 113,500 people out of Afghanistan in the past two weeks, but tens of thousands who want to go will be left behind.

A US official told Reuters on Saturday there were fewer than 4,000 troops left at the airport, down from 5,800 at the peak of the evacuation mission. Pentagon spokesperson John Kirby told reporters some troops had been withdrawn but declined to say how many remained.

The Taliban official told Reuters the group had engineers and technicians ready to take charge of the airport.

"We are waiting for the final nod from the Americans to secure full control over Kabul airport as both sides aim for a swift handover," the official said on condition of anonymity.

The Western security official said crowds at the airport gates had diminished after a specific warning from the US government of another attack by militants after a suicide bombing outside the airport on Thursday.

The explosion killed scores of Afghans and 13 American troops outside the gates of the airport, where thousands of Afghans had gathered to try to get a flight out since the Taliban returned to power.

The United States said on Friday it killed two militants belonging to the Islamic State — enemies of both the West and Afghanistan's new Taliban rulers — which had claimed responsibility for the attack.

Biden had vowed to hunt down the perpetrators of the explosion and said the strike was not the last.

Taking over

The Taliban condemned the late-night US drone strike, which took place in Nangarhar province, an eastern area that borders Pakistan.

"The Americans should have informed us before conducting the airstrike. It was a clear attack on Afghan territory," a Taliban spokesman told Reuters, adding that two women and a child were wounded in the attack.

The Taliban have said they have arrested some suspects involved in the airport blast.

Spokesman Zabihullah Mujahid said on Saturday the Taliban would take over the airport "very soon" after US forces withdraw and announce a full cabinet in the coming days.

Mujahid told Reuters the group had appointed governors and police chiefs in all but one of Afghanistan's 34 provinces and would act to solve the country's economic problems.

The Taliban, facing the loss of billions of dollars of aid for the country, appealed to the United States and other Western nations to maintain diplomatic relations after withdrawing. Britain said that should happen only if the Taliban allow safe passage for those who want to leave and respect human rights.

US military and allied countries' flights carried fewer people on Saturday as Washington prepared to end its mission.

The last British flight evacuating civilians from Afghanistan left Kabul on Saturday. British troops would take small numbers of Afghan citizens with them as they leave this weekend, a defence ministry spokesperson said. Armed forces chief Nick Carter said hundreds of people who had worked with Britain would not make it through.

While Kabul's airport has been in chaos, the rest of the city has been generally calm. The Taliban have told residents to hand over government equipment including weapons and vehicles within a week, the group's spokesman said.

The airport attack added fuel to criticism Biden faced at home and abroad for the chaos after Afghanistan's government and military collapsed before a lightning Taliban advance. He has defended his decisions, saying the United States long ago achieved its rationale for invading in 2001.