U.S. 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.

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

United States forces helping to evacuate Afghans desperate to flee Taliban rule braced for more attacks on Friday after an Islamic State suicide bomber killed scores of civilians and 13 U.S. soldiers outside the gates of Kabul airport.

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

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". U.S. officials also blamed the group and vowed retribution.

General Frank McKenzie, head of U.S. Central Command, said U.S. 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."

U.S. 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," Biden said during televised comments from the White House.

Video taken in the aftermath of the attack showed corpses in a sewage 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.

A U.S. Central Command spokesperson said 18 soldiers injured in the attack were "in the process of being aero-medically evacuated from Afghanistan on specially equipped C-17s with embarked surgical units".

A Taliban official said at least 28 members of the militant group were killed in the attack.

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

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 Centre 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 around 1,000 U.S. 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 U.S. 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 U.S. troops killed in Afghanistan in a single incident since 30 personnel died when a helicopter was shot down in 2011.

The U.S. 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 pull out.