U.S. forces helping to evacuate Afghans desperate to flee Taliban rule were on alert for more attacks on Friday after at least one Islamic State suicide bomber killed 85 people including 13 U.S. 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, although a Taliban spokesman later denied that any of their fighters guarding the airport perimeter had been killed.

The U.S. military said 13 of its service members were killed in what it described as a complex attack.

Islamic State (ISIS), an enemy of the Islamist Taliban as well as the West, said one of its suicide bombers targeted "translators and collaborators with the American army".

It was not clear if suicide bombers detonated both blasts or if one was a planted bomb. It was also not clear if ISIS gunmen were involved in the attack or if the firing that followed the blasts was Taliban guards firing into the air to control crowds.

General Frank McKenzie, head of U.S. Central Command, said U.S. commanders were watching for more attacks by Islamic State, including possibly rockets or car-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. He says the United States 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 that year.

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 after 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."

Several thousand people were back at the airport fence on Friday, according to video posted on social media.

Staking a claim

British Defence Secretary Ben Wallace said the threat of attacks would increase as Western troops got closer to completing the huge airlift and leaving.

"The narrative is always going to be, as we leave, certain groups such as ISIS will want to stake a claim that they have driven out the U.S. or the UK," Wallace told Sky News. He also vowed action against ISIS wherever it manifests itself.

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

A Taliban official lamented the number of its members killed, saying they had lost more men that the Americans, but Taliban spokesman Zabihullah Mujahid later said no Taliban were killed in the twin blasts.

Western countries fear that the Taliban, who once sheltered Osaka bin Laden's al Qaeda before it was ousted from power by the U.S.-led 2001 invasion, 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.

100,000 taken out in airlift

The United States will press on with evacuations despite the threat of further attacks, McKenzie said, noting that there were still about 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, according to 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.

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 pullout.