There remain "credible threats" against the major airlift operation at Kabul airport, the Pentagon said today, a day after a suicide attack on a crowd trying to flee Taliban-controlled Afghanistan killed at least 85 people, including 13 US servicemen.

The blast claimed by the Islamic State terror group targeted US forces, but hit hardest the mass of people fearing life under the Taliban who converged on the airport in a desperate bid to board a flight out.

At least 72 people were killed, including the 13 American troops, according to various authorities, in an attack that came after a chorus of warnings from foreign governments of an imminent threat.

"We still believe there are credible threats... specific, credible threats," said US military spokesman John Kirby today.

There are more than 5,000 people inside the airport left to evacuate, a US General said, stressing that airlifts would continue "until the last moment".

Determined to end two decades of war in Afghanistan, US President Joe Biden has insisted that he would stick to his August 31 deadline to end the airlift.

Mr Biden, under enormous pressure over his administration's handling of the Afghan crisis, vowed to punish those responsible for the blast.

"We will not forgive. We will not forget. We will hunt you down and make you pay," he said.

The Pentagon also clarified that there was one explosion, and not two as it previously reported.

About 105,000 people have been flown out of the country since August 14, the day before the Taliban swept to power, according to the White House.

Some Western allies, including Britain and Spain, announced an end to their airlifts on Friday, following other nations such as Canada and Australia earlier in the week.

Many have repeatedly warned that it will be impossible to get all at-risk Afghans out by Tuesday.

Refugee exodus

The Taliban have promised a softer brand of rule compared with their first stint in power, which ended in 2001 when the United States hit back following the September 11 attacks.

But many Afghans fear a repeat of their brutal interpretation of Islamic law, as well as violent retribution for working with foreign militaries, Western missions or the previous US-backed government.

Today morning, some evacuation flights resumed, with queues of people seen lining up on the tarmac but there were no more crowds near the sites of the blast, according to AFP reporters.

The Taliban had also blocked off the main routes to the airport as well as side roads.

The United Nations said today it was bracing for a "worst-case scenario" of up to half a million more refugees from Afghanistan by the end of 2021.

The Taliban has said that Afghans with legal documents will be allowed to travel abroad after the airport reopens to commercial flights.

'More extreme'

The Taliban have allowed US-led forces to conduct the airlift while they finalise plans for their government to be announced as soon as the American troops have left.

But the Islamic State terrorists, rivals of the Taliban with a record of barbaric attacks, were intent on capitalising on the chaos in Kabul.

In recent years, the Islamic State's Afghanistan-Pakistan chapter has been responsible for some of the deadliest attacks in those countries.

It has massacred civilians at mosques, shrines, public squares and even hospitals.

"These are people that are even more extreme than the Taliban and are basically at war with the Taliban. So it is a horribly complex situation," Australian Defence Minister Peter Dutton said.

Turkish President Recep Tayyip Erdogan said today that Ankara was assessing an offer to run operations at the Kabul airport, with security to be provided by the Taliban.

Terror, devastation

The blast hit people trying to reach access gates at the airport, creating scenes of terror and devastation.

Immediately after the first, one man held a semi-conscious victim by the elbow, trying to stop his head from slipping beneath the surface of the fetid water in a canal along the perimeter of the airport.

"I will never, ever want to go (to the airport) again. Death to America, its evacuation and visas," Milad, who was near the first blast with his wife and three children, told AFP.

The Italian NGO Emergency said the hospital it operates in Kabul had been overwhelmed by more than 60 casualties, 16 of whom were pronounced dead on arrival.

The injured "could not speak, many were terrified, their eyes totally lost in emptiness, their gaze blank", the hospital's medical coordinator Alberto Zanin said in a post on the group's Twitter account.

'Bear responsibility'

The attacks led to the worst single-day death count for the US military in Afghanistan since 2011.

A clearly shaken Mr Biden went before TV cameras to address the American people, describing the killed US troops as heroes.

Asked by a reporter if he bore any responsibility for the deaths, he said: "I bear responsibility fundamentally for all that's happened of late."

The Taliban, which condemned the blast, emphasised that it took place in an area under US military control.