U.S. forces helping evacuate Afghans desperate to flee Taliban rule were on alert for more attacks on Friday after an Islamic State suicide bombing outside Kabul airport killed at least 92 people, including 13 U.S. service members.

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

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," Kirby told reporters in Washington. "We're monitoring these threats, very, very specifically, virtually in real time."

U.S. 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 there.

About 4,200 people were evacuated from Kabul during a 12-hour period on Friday, the White House said. Earlier in the day U.S. officials said a total of approximately 5,100 Americans had been evacuated, with about 500 more waiting to leave.

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

Some U.S. media including the New York Times cited local health officials as saying up to 170 people, not including the U.S. troops, were killed in Thursday's attack.

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

Taliban forces have taken up positions in Kabul airport and are ready to take full control as early as this weekend, as soon as U.S. forces leave, two senior Taliban figures said. One senior commander said Taliban 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 Taliban official said.

Kirby disputed the Taliban account, telling reporters that the military section of the airport remains under U.S. 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 U.S. military control," Kirby said.

The United States expects some ongoing engagement with the Taliban will be necessary after the withdrawal to facilitate further evacuations, White House Press Secretary Jen Psaki said.

"The reality is, the Taliban control large swathes of Afghanistan, including the areas surrounding the perimeter of the airport," she told reporters. "So by necessity, that is our option."

SINGLE SUICIDE BOMBER

Islamic State (ISIS), an enemy of the Islamist Taliban 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 number of Afghans killed has risen to 79, a hospital official told Reuters on Friday, adding that more than 120 were wounded. A Taliban official said the dead included 28 Taliban members, although a spokesman later denied any such fighters had been killed.

The attack underlined the realpolitik facing Western powers in Afghanistan: Engaging with Taliban forces who they have long fought may be their best chance to prevent the country 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.

Biden said on Thursday he has ordered the Pentagon to plan how to strike ISIS-K, the Islamic State affiliate that claimed responsibility. Asked on Friday if Biden sought to capture and put on trial those responsible, Psaki said "I think he made it clear yesterday that he does not want them to live on the earth anymore."

Biden was already facing strong criticism at home and abroad for the chaos surrounding the troop withdrawal and evacuations. As the Taliban rapidly advanced to Kabul amid the pullout, Afghanistan's Western-backed government and military collapsed. Biden has defended his decisions, saying the United States long ago achieved its rationale for invading the country in 2001.

The U.S.-led invasion toppled the then-ruling Taliban, punishing them for harboring al Qaeda militants who masterminded the Sept. 11 attacks on the United States 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 Taliban 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.

Taliban 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 Organization 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 wounded.

Up to half a million Afghans could flee their homeland by year-end, the U.N. refugee agency UNHCR said, appealing to all neighboring 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 Taliban have asked all women healthcare workers to return to work, a spokesman said, as trained and educated Afghans flee the country.