US forces helping to evacuate Afghans desperate to flee Taliban rule braced for more attacks on Friday after an Islamic State suicide bomber killed 85 people, including 13 US soldiers outside the gates of Kabul airport.

Two blasts and gunfire rocked the area outside the airport on Thursday evening, witnesses said. Video shot by Afghan journalists showed dozens of bodies strewn around a canal on the edge of the airport.

A health official and a Taliban official said the toll of Afghans killed had risen to 72, including 28 Taliban members. The US military said 13 of its service members were killed.

Islamic State (ISIS), an enemy of the Taliban as well as the West, 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, adding that some intelligence was being shared with the Taliban and that he believed "some attacks have been thwarted by them."

US forces are racing to complete their withdrawal from Afghanistan by an Aug 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 Sept 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.

Video taken in the aftermath of the attack showed corpses in a waste water canal by the airport fence, 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 witness. "That little water flowing in the sewage canal had turned into blood."

Zubair, a 24 year-old civil engineer, said he was close to a suicide bomber who detonated explosives.

"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.

'Chaotic evacuation'

A US Central Command spokesperson said 18 soldiers wounded in the attack were "in the process of being aeromedically evacuated from Afghanistan on specially equipped C-17s with embarked surgical units".

A Taliban official lamented the number of Taliban members killed in the ISIS attack.

"We have lost more people than the Americans in the airport blast," a Taliban official said, adding that the Taliban was "not responsible for the chaotic evacuation plan prepared by foreign nations".

A NATO country diplomat in Kabul said all foreign forces were aiming to evacuate their citizens and embassy employees by Aug 30.

The Taliban would tighten security around the airport, said the diplomat who declined to be identified.

"Security is their responsibility," the diplomat said, adding that the Taliban should investigate the Islamic State network.

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.

ISIS-K was initially confined to areas on the border with Pakistan but has established a second front in the north of the country. The Combating Terrorism Center at West Point says ISIS-K includes Pakistanis from other militant groups and Uzbek extremists in addition to Afghans.

Airlift continues

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

The pace of evacuation flights had accelerated on Friday and American passport holders had been allowed to enter the airport compound, said a Western security official stationed inside the airport.

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.