KABUL (REUTERS) - US forces helping to evacuate Afghans desperate to flee Taleban rule were on alert for more attacks on Friday (Aug 27) after an Islamic State in Iraq and Syria (ISIS) suicide bombing killed at least 92 people, including 13 US service members, just outside Kabul airport.

The White House said the next few days of the ongoing US evacuation operation are likely to be the most dangerous. The Pentagon said about 111,000 people have been taken out of Afghanistan in the past two weeks.

Pentagon spokesman John Kirby said the United States believes there are still "specific, credible" threats against the airport after the bombing at one of its gates.

"We certainly are prepared and would expect future attempts," Mr Kirby told reporters in Washington. "We're monitoring these threats, very, very specifically, virtually in real time."

"US citizens who are at the Abbey gate, East gate, North gate or the New Ministry of Interior gate now should leave immediately," the US Embassy in Kabul said in a security alert on Friday.

"Because of security threats at the Kabul airport, we continue to advise US citizens to avoid traveling to the airport and to avoid airport gates," the embassy said.

US and allied forces are racing to complete evacuations of their citizens and vulnerable Afghans and to withdraw from Afghanistan by an Aug 31 deadline set by President Joe Biden after two decades of American military presence in Afghanistan.

In the past day, another 12,500 people were evacuated, including more than 300 American citizens, US officials said.

A total of approximately 5,100 Americans have been evacuated, US officials added, with about 500 more waiting to leave.

While thousands have been evacuated, they are far outnumbered by those who could not get out.

Some US media outlets, including The New York Times, cited local health officials as saying that up to 170 people, not including the US troops, had died in the attack.

Thousands of people have gathered outside the airport to try to get onto evacuation flights since the Taleban took control of Afghanistan on Aug 15. The attack was the deadliest incident for US troops in Afghanistan in a decade.

Taleban forces have taken up positions in Kabul airport and are ready to take full control as early as this weekend, as soon as US forces leave, two senior Taleban figures said. One senior commander said Taleban forces had taken over most of the airport, "just not a small part where the Americans still are".

"As soon as the Americans leave, they just have to give us the signal and we will then take over," a second Taleban official said.

Mr Kirby disputed the Taleban account, telling reporters that the military section of the airport remains under US control.

"They are not in charge of any of the gates. They are not in charge of any of the airport operations. That is still under US military control," he said.

The US expects some ongoing engagement with the Taleban will be necessary after the withdrawal to facilitate further evacuations, White House press secretary Jen Psaki said.

Single suicide bomber

The ISIS, an enemy of the Islamist Taleban as well as the West, has claimed responsibility for the attack, which the Pentagon said on Friday was carried out by one suicide bomber at an airport gate, not two as it earlier stated. The Pentagon had said on Thursday a second bomb had exploded at a nearby hotel.

The number of Afghans killed has risen to 79, a hospital official told Reuters on Friday, adding that more than 120 were wounded. A Taleban official said the dead included 28 Taleban members, although a spokesman later denied any such fighters had been killed.

The attack underlined the realpolitik facing Western powers in Afghanistan: Engaging with Taleban forces that they have long sought to fend off may be their best chance to prevent the country from becoming a breeding ground for Islamist militancy.

The United Nations Security Council condemned the bombing as "especially abhorrent" for targeting civilians trying to flee the country.

Mr Biden said on Thursday he has ordered the Pentagon to plan how to strike ISIS-K, the Khorasan affiliate that claimed responsibility.

Asked on Friday if Mr Biden sought to capture and put on trial those responsible, Ms Psaki said: "I think he made it clear yesterday that he does not want them to live on the earth any more."

Mr Biden was already facing strong criticism at home and abroad for the chaos surrounding the troop withdrawal and evacuations.

As the Taleban forces rapidly advanced to Kabul amid the pullout, Afghanistan's Western-backed government and military collapsed.

Mr Biden has defended his decisions, saying the United States long ago achieved its rationale for invading the country in 2001.

The US-led invasion toppled the then ruling Taleban, punishing it for harbouring Al-Qaeda militants who masterminded the Sept 11 attacks on the US that year.

Most of the more than 20 allied countries involved in airlifting Afghans and their own citizens out of Kabul said they had completed evacuations by Friday.

The Taleban said that Afghans with valid documents would be able to travel freely in future at any time - comments aimed at calming fears that the movement planned harsh restrictions on freedom.

Taleban guards blocked access to the airport on Friday, witnesses said. "We had a flight but the situation is very tough and the roads are blocked," said one man on an approach road.

Medical supplies will run out within days in Afghanistan, the World Health Organisation said, adding that it hopes to establish an air bridge into the northern city of Mazar-i-Sharif with the help of Pakistan.

Pakistani officials told Reuters that at the Torkham border crossing, Pakistani security forces had opened fire on a group of people trying to illegally enter Pakistan, adding that two Afghans were killed and two others wounded.

Those killed on Thursday included two British nationals and the child of a third British national, British Foreign Minister Dominic Raab said.

Up to half a million Afghans could flee their homeland by year-end, the United Nations refugee agency UNHCR said, appealing to all neighbouring countries to keep their borders open.

There are also growing worries Afghans will face a humanitarian emergency with the coronavirus spreading and shortages of food and medical supplies looming.

The Taleban has asked all women healthcare workers to return to work, a spokesman said, amid mounting pressure on public services as trained and educated Afghans flee the country.