US troops helping to evacuate Afghans desperate to flee Taliban rule braced for more attacks on Friday after Islamic State struck the crowded gates of Kabul airport, killing scores of civilians and at least 13 US troops.

Kabul health officials were quoted as saying 60 civilians were killed in the attack on Thursday. Video shot by Afghan journalists showed dozens of bodies strewn around a canal on the edge of the airport. At least two blasts and gunfire rocked the area, witnesses said.

Islamic State (ISIS) said one of its suicide bombers targeted "translators and collaborators with the American army". US officials also blamed the group and vowed retribution.

General Frank McKenzie, head of US Central Command, said US commanders were on alert for more attacks by Islamic State, including possibly rockets or vehicle-borne bombs targeting the airport.

"We're doing everything we can to be prepared," he said.

US forces are racing to complete their withdrawal from Afghanistan by an August 31 deadline set by President Joe Biden, who says the United States had long ago achieved its original rationale for invading the country in 2001: to root out al Qaeda militants and prevent a repeat of the September 11 attacks on the United States.

Biden said he had ordered the Pentagon to plan how to strike ISIS-K, the Islamic State affiliate that claimed responsibility.

"We will not forgive. We will not forget. We will hunt you down and make you pay," Biden said during televised comments from the White House.

Corpses were in the canal by the airport fence, video from the scene showed, some being fished out and laid in heaps while wailing civilians searched for loved ones.

"I saw bodies and body parts flying in the air like a tornado blowing plastic bags," said one Afghan who had been trying to reach the airport. "That little water flowing in the sewage canal had turned into blood."

Zubair, a 24 year-old civil engineer, who had been trying for nearly a week to get inside the airport, said he was close to a suicide bomber who detonated explosives at the gate.

"Men, women and children were screaming. I saw many injured people – men, women and children – being loaded into private vehicles and taken toward the hospitals," he said.

A Taliban spokesman described the attack as the work of "evil circles" who would be suppressed once foreign troops leave.

Western countries fear that the Taliban, who once sheltered Osama bin Laden's al Qaeda, will allow Afghanistan to turn again into a haven for militants. The Taliban say they will not let the country be used by terrorists.

Airlift continues

The United States would press on with evacuations despite the threat of further attacks, McKenzie said, noting that there were still around 1,000 US citizens in Afghanistan.

In the past 12 days, Western countries have evacuated nearly 100,000 people. But they acknowledge that thousands will be left behind when the last US troops leave at the end of the month.

Several Western countries said the mass airlift of civilians was coming to an end and announced their last remaining troops had left the country.

The American casualties in Thursday's attack were believed to be the most US troops killed in Afghanistan in a single incident since 30 personnel died when a helicopter was shot down in 2011.

The US deaths were the first in action in Afghanistan in 18 months, a fact likely to be cited by critics who accuse Biden of recklessly abandoning a stable and hard-won status quo by ordering an abrupt pullout.