A US military aircraft takes off at the Hamid Karzai International Airport in Kabul, Afghanistan, on Saturday. AP

Suicide bomb threats hung over the final phase of the US military's airlift operation from Kabul on Sunday as US President Joe Biden has vowed to keep up airstrikes against the Daesh group whose suicide bombing at the Kabul airport killed scores of Afghans and 13 American service members.

US President also warning another attack was highly likely before the evacuations end.

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What had already been a chaotic and desperate evacuation turned bloody on Thursday when a suicide bomber from the local chapter of the Daesh group targeted US troops stopping huge crowds of people from entering the airport.

This combination of photos shows twelve service members killed in the Kabul airport bombing in Afghanistan. AP

More than 100 people died in the attack, including 13 US service personnel, slowing down the airlift ahead of Biden's deadline for evacuations to end by Tuesday.

More than 112,000 people have fled Afghanistan via the massive US-led evacuation since the Taliban movement swept back into power a fortnight ago, and the operation is winding down despite Western powers saying thousands may be left behind.

The Pentagon said the remaining contingent of US forces at the airport, now numbering fewer than 4,000, had begun their final withdrawal ahead of Biden's deadline for ending the evacuation on Tuesday.

Volunteers and medical staff bring an injured man on a stretcher to a hospital in Kabul on Thursday. File/AFP

After getting briefed on a US drone mission in eastern Afghanistan that the Pentagon said killed two members of the Daesh group's Afghanistan affiliate early Saturday, Biden said Saturday the extremists can expect more.

"This strike was not the last,” Biden said in a statement. "We will continue to hunt down any person involved in that heinous attack and make them pay.”

Taliban fighters stand guard at a checkpoint near the gate of Hamid Karzai international Airport in Kabul. AP

The evacuation proceeded as tensions rose over the prospect of another Daesh attack. The State Department issued a new security alert early Sunday morning Kabul time instructing people to leave the airport area immediately "due to a specific, credible threat.”

"Our commanders informed me that an attack is highly likely in the next 24-36 hours,” Biden said, adding that he has instructed them to take all possible measures to protect their troops, who are securing the airport and helping bring onto the airfield Americans and others desperate to escape Taliban rule.

Western allies that helped with the airlift have mostly already ended their flights, with some voicing despair at not being able to fly out everyone at risk.

The head of Britain's armed forces, General Sir Nick Carter, told the BBC it was "heartbreaking" that "we haven't been able to bring everybody out".

At the airport, gone are the crowds of thousands mobbing the perimeter, hoping to be let through and allowed onto a plane.

The Taliban have now sealed off roads leading to the facility and are only letting sanctioned buses pass.