Chaos at Kabul airport amid struggle to flee

President Joe Biden is set to address the nation as the crisis intensifies.

The Pentagon also announced Monday that more soldiers from 82nd Airborne will head to Kabul. Now, 6,000 U.S. troops will soon be in the country's capital. There are 2,500 there already.

Earlier Monday, a U.S. official told Reuters that U.S. troops had fired shots into the air to prevent civilians from running onto the tarmac at Hamid Karzai International Airport in Kabul, where scenes on the ground show an increasingly panicked people.

"The crowd was out of control. The firing was only done to defuse the chaos and prevent the crowd from storming the runway," the official told Reuters.

The State Department and Pentagon issued a joint statement Sunday night to say they would "accelerate" evacuation flights for Afghans who served the U.S. mission and take over air traffic control at the airport after the U.S. Embassy in Kabul was "successfully" evacuated. By Monday morning, U.S. troops had reportedly established large barriers and roadblocks on the streets near the airport in an effort to both slow and control the fleeing population.

A security alert Monday from the U.S. Embassy warned American citizens that are set to be evacuated but have not yet left the country to still shelter in place -- until they "have been informed by email that departure options exist."

White House National Security Adviser Jake Sullivan stood by the administration's decision to withdraw troops by Aug. 31 on ABC's "Good Morning America" Monday and placed blame for the speed of the Taliban takeover on the Afghan army for its decision "not to step up and fight for their country."

"What the president was not prepared to do was enter a third decade of conflict, flowing in thousands of more troops, which was his only other choice, to fight in the middle of a civil war that the Afghan army wouldn't fight for itself," Sullivan said. "He would not do that to America's men and women or their families, and that is why he made the decision to withdraw U.S. forces from Afghanistan this year."

As with Pentagon officials, Sullivan would not say that the takeover took the administration by surprise but did acknowledge it "certainly unfolded at an unexpected speed." Presented with the fact that thousands of allies appear stranded in the country, Sullivan said the U.S. had planned for a "wide range of contingencies," though he didn't offer more specifics on timing or numbers of people that will be moved.

"We believe that we can effectuate an ongoing evacuation of American citizens, of Afghans who worked for us, including interpreters and translators and others vulnerable Afghans at risk. We're working to do that by securing the airport today and in the days ahead by taking people out one flight at a time, flight after flight," Sullivan said.

Sullivan insisted Afghanistan presented the U.S. with an opportunity to prove "that we can fight terrorism effectively without having a large military footprint on the ground."

He also said the American people can expect to hear from the president "soon" as President Joe Biden faces criticism from lawmakers on both sides of the aisle and the public for how the withdrawal has played out.

The increase of service members in Kabul follows Gen. Frank McKenzie, the commander of U.S. Central Command, meeting with representatives of the Taliban earlier Sunday in Doha, Qatar, to inform them not to interfere with the U.S. mission at the airport, according to a U.S. official.

As the Taliban advanced last week, the U.S. State Department announced it was reducing its staff levels at the U.S. Embassy in Kabul and the Pentagon began sending in U.S. troops to help facilitate departures of Americans and Afghan allies.

Pentagon details 'shooting at US forces,' deploying more troops to Kabul

Pentagon spokesman John Kirby provided an update to reporters on the worsening crisis in Afghanistan on Monday afternoon.

He said U.S. forces at the Kabul airport, along with troops from Turkey and other countries, are working "to clear the area of people" and "restore operations at the airport."

"We do not know how long this will take certainly seeing all the dramatic video coming from the airport today and we obviously don't want anyone else to get hurt. So we're going to work methodically in coming hours to restore a safe and secure environment so that air operations can resume," he said in an off-camera briefing.

Kirby said there were security incidents at the airport Sunday night "involving armed individuals, shooting at U.S. forces," and that the U.S. responded in self-defense with fatal fire.

"I want to reiterate that while our mission is not offensive, forces have the inherent right of self-defense, and they will respond accordingly to threats and attacks to separate incidents. U.S. forces did respond to hostile threats that resulted in the death of two armed individuals on the support for vulnerable or at-risk Afghan citizens," he said, adding that the individuals were Taliban.

Kirby confirmed that Centcom's Gen. McKenzie met Sunday with Taliban officials in Doha, but would not provide any other details about a Taliban commitment.

"He was firm and clear with Taliban leaders that any attack on our people, or our operations at the airport would be met, swiftly with a very forceful response," Kirby said.

Late Sunday, the Pentagon approved a State Department request to help house up to 22,000 Afghan interpreters and their families (SIV's) in the U.S., but Kirby acknowledged it will "take some time to build it out."

The world reacts

An emergency session of the United Nations will be held Monday, and leaders around the globe have been issuing responses to the Taliban takeover of Afghanistan.

U.N. Secretary-General António Guterres said in remarks he's "particularly concerned by accounts of mounting human rights violations against the women and girls of Afghanistan who fear a return to the darkest days."

British Defense Secretary Ben Wallace spoke about the race against time to evacuate Brits and Afghans who helped the military when he became overcome with emotion after saying that some people who served the U.K. will not come back.

When asked in an interview with British radio station LBC why he was taking it so personally, he said, "Because I'm a soldier. It's sad, and the West has done what it's done, and we have to do our very best to get people out and stand by our obligations."

Wallace announced Monday that the U.K. is working on plans to allow Afghan asylum seekers to enter Britain without a passport as the situation on the ground deteriorates.

Russia's foreign ministry, meanwhile, said Monday it believes the situation in Afghanistan is "stabilising" and that it has established contact with the "new authorities" in Kabul.

Russian officials have said the Taliban had promised no Russians would be harmed and that the group's fighters have now taken Russia's embassy under protection in Kabul. Russia has not evacuated its embassy so far, though it has pulled out some staff.

In China, official statements laid the groundwork for Beijing recognizing a Taliban government.

Spokesperson Hua Chunying noted that the Taliban said Sunday that the "war in Afghanistan is over" and that they will work to establish an inclusive government and ensure the safety of foreign missions in Afghanistan. Chunying said China expects these statements to be implemented in order to ensure a smooth transition and curb terrorist and criminal acts, so the Afghan people can avoid war and rebuild their country.

China has been wanting to expand their Belt and Road infrastructure initiative into Afghanistan but the U.S.-backed government had been reluctant to commit. China is connected to Afghanistan by a sliver of land called the Wakhan Corridor, which has historically been a well-traveled trade route. It connects to China's Xinjiang region, and if it were opened, it would provide a better route from Kashgar, China, to Peshawar, Pakistan.

ABC News' Julia Macfarlane, Patrick Reevell, Karson Yiu, Cindy Smith, Sarah Kolinovsky and Celia Darrough contributed to this report.