Horrifying scenes of desperate Afghans, lunging for the wheels of a U.S. military aircraft as it taxied for takeoff from Kabul airport Monday, were juxtaposed with an eerie tension in much of the rest of Afghanistan’s capital, where many huddled behind closed doors as Taliban militants patrolled the streets.

In Washington, President Biden called the scenes “gut-wrenching.” Human remains were discovered in the wheel well of one of the military planes after it landed in a third country, according to several people familiar with the situation, who spoke on the condition of anonymity because they were not authorized to discuss the matter.

Speaking in an address from the White House, Biden defended in forceful tones his order in April to begin a full withdrawal of U.S. troops from Afghanistan.

“I stand squarely behind my decision,” Biden declared, saying that after 20 years of combat in Afghanistan, “I’ve learned the hard way that there was never a good time to withdraw U.S. forces.” The result, he said, would have been the same five or 15 years from now.

President Biden addressed the Taliban’s takeover of Afghanistan during remarks on Aug. 16. Here’s his speech in less than 3 minutes. (Blair Guild/The Washington Post)

Blaming Afghan political and military leaders who “gave up” in the face of a Taliban offensive that swept the country in little more than a week, he acknowledged that the capitulation “did unfold more quickly than we anticipated.”

“I know my decision will be criticized, but I would rather take all that criticism” than violate a campaign pledge and pass on an unwinnable war to yet another U.S. president, Biden said.

The criticism was flowing freely by the time Biden spoke after a weekend at Camp David, where one Republican lawmaker charged he had cowered while “the Taliban are humiliating America.” Sen. Ben Sasse (R-Neb.) wrote the critique for the conservative National Review, under the headline “Worse Than Saigon.”

The White House distributed talking points to Democrats through the office of House Speaker Nancy Pelosi (D-Calif.). Some, while expressing alarm over what many called the “tragedy” in Kabul, echoed Biden’s comments.

“President Joe Biden is cleaning up a mess left by three presidents before him,” Sen. Ben Cardin (D-Md.) said in a statement. “George W. Bush failed to complete the mission in Afghanistan and diverted resources to a new mission in Iraq unrelated to 9/11. President Barack Obama wanted to pull troops out, but couldn’t find a way.”

“President Donald Trump announced a withdrawal, invited the Taliban to Camp David on the eve of the 9/11 anniversary and cut secret deals with them leading directly to what we are seeing in Afghanistan today,” Cardin’s statement said.

In speeches and statements from world capitals and the United Nations, official reactions ranged from trepidation, to warnings to the Taliban, to wait-and-see shrugs. In Berlin, German Chancellor Angela Merkel called the unfolding drama “bitter, dramatic and terrifying,” while British Defense Secretary Ben Wallace appeared close to tears as he told a London radio interviewer that the inability to quickly evacuate all Afghans eligible to come to Britain was a “really deep part of regret for me.” A number of NATO governments have sent their own aircraft and troops to pick up diplomats, citizens and Afghan employees.

While nearly all global powers had signed on last week to international statements warning against a Taliban takeover and pledging to cut off the group from recognition and aid, on Monday at least some referred to the militants’ apparent restraint thus far, at least in Kabul, as a reason to reconsider.

“According to our reports, the Taliban have already sought to bring public order and have also confirmed security guarantees for civilians and for the diplomatic missions and staffs,” Russia’s ambassador to the United Nations, Vasily Nebenzya, told a special session of the Security Council. “In these circumstances, the Russian embassy in Kabul is continuing to operate normally,” he said.

State Department spokesman Ned Price said Monday that the United States was still “taking stock” of the Taliban’s takeover of Afghanistan and would decide whether to recognize its rule only if the militant group demonstrated a willingness to govern inclusively and prohibit terrorists from operating on its soil.

U.N. Secretary General António Guterres, who attended the Security Council session, urged the international community not to “abandon” the people of Afghanistan.

Biden, after finishing his midafternoon address, returned to Camp David.

Whatever immediate credit the president might have earned for the all-but-completed withdrawal of U.S. forces from Afghanistan was diluted by last week’s new authorization to deploy at least 6,000 troops to secure the airport and evacuate U.S. and allied diplomats and citizens, as well as Afghans whose work for the Americans over the past two decades puts them at risk.

While troops continued to arrive Monday, the Taliban, now securely in charge of the rest of Kabul, continued to stay away from the airport, where the U.S. force controlled both the civilian and military sides and took over air traffic control.

But the spontaneous arrival there beginning Sunday of what appeared to be thousands of Afghans seeking to escape the country severely complicated the evacuation mission. Commercial flights, initially seen as a way to hasten the departures, were canceled as the gathering crowds spilled onto the runway where military cargo planes were trying to land, load evacuating passengers and take off.

At least seven people were killed at the airport, the Associated Press reported, although the causes of the deaths were unclear.

Pentagon spokesman John Kirby said U.S. troops came under fire twice at the airport Monday, with a preliminary report that one service member was wounded. Another U.S. official, speaking on the condition of anonymity because of the sensitivity of the issue, said the service member appeared to be in stable condition. The two gunmen were killed, a second U.S. official said.

The U.S. military made at least one flight out of Kabul during the day. But even that was harrowing. In a video distributed widely online, a hulking Air Force C-17 gathered speed as Afghans ran alongside the plane and its jet engines. Some people climbed on. U.S. officials said more than 600 people were aboard, a detail first reported by Defense One.

A second viral video appeared to show two people falling to their deaths from an airborne plane. The first official said that the U.S. military was assessing the situation and that the fall “absolutely happened.” It is believed that they were Afghans who climbed aboard the landing gear and attempted to stow away as the plane took off. People familiar with the situation said the pilots declared an emergency when they could not put their landing gear up. The crew diverted and landed in a nearby third country, and some human remains were found in the wheel well when it was inspected, they said.

Kirby, speaking at a Pentagon briefing, said U.S. troops worked with Turkish forces and some other international troops to secure the airport and clear the runway. Military flights resumed later in the day.

As the chaos unfolded, Defense Secretary Lloyd Austin, acting on Biden’s deployment order last week, authorized an additional Army battalion with the 82nd Airborne Division to join the Americans on the ground, part of the 6,000 troops expected to be there later this week.

On Monday, Kirby said, only about 2,500 troops were currently in Afghanistan, underscoring how the military was still catching up to the crisis.

The situation was challenging in part because of the layout of the airport.

Thousands of Americans — including a small group of diplomats remaining behind as more than 1,000 American Embassy workers waited to evacuate — are operating from a secure military encampment on its northern side that U.S. troops call “HKIA,” short for Hamid Karzai International Airport. The runway is between the military outpost and the commercial airport terminal to the south, and desperate Afghans have breached the runway from that side, Kirby said.

Just a short distance from the airport, Taliban commanders were sending armed insurgents to direct traffic, prevent looting, encourage fearful Afghans to go back to work and monitor a 9 p.m. curfew, in an apparent effort to demonstrate their governing capabilities and avoid an international backlash, at least until foreigners depart.

When the Taliban arrived in Kabul on Sunday, with armed fighters suddenly appearing in the streets, some residents panicked, rushing to banks to withdraw their money, shuttering their shops and gathering their families. Others, according to numerous videos posted online and on social media, crowded around the insurgents, denouncing the government of Ashraf Ghani and taking selfies.

In provincial towns and cities overtaken in recent days, there were reports of a return to the brutal ideology of the Taliban of the 1990s, including harsh dictates, shuttered schools, orders to women and girls to stay at home, and worse.

But in much of downtown Kabul, Monday was mostly peaceful, a stark contrast to the chaos at the airport. Stores were largely closed, but a few people still went about their business.

Taliban fighters made their way through the capital in pickup trucks bearing the group’s white flag. Some set up checkpoints, while others posed for pictures at well-known landmarks.

But it was unclear to many whether the apparent restraint by the Taliban was a tactical move to allow foreigners to exit and lull the international community, or a longer-term policy. Many Kabul residents, speaking to reporters by telephone or online, reported a sense of tense anticipation.

Tolo News, Afghanistan’s biggest nationwide television broadcaster, said Monday that militants had entered its compound in Kabul, “checked the weapons of the security staff, collected government-issued weapons and agreed to keep the compound safe.”

“There was no improper treatment of staff members,” the organization reported on its English-language website.

The network has continued to broadcast and publish online. On its homepage, Tolo reported the chaos at the airport, along with statements by foreign leaders warning the Taliban against violence.

In Doha, Qatar, where Ghani’s government had grudgingly participated in U.S.-midwifed negotiations with the Taliban until last week, Marine Gen. Kenneth “Frank” McKenzie, the chief of U.S. Central Command, met with Taliban political leaders who remain there. Defense officials declined to provide many specifics about their conversation, but Kirby said the general conveyed that if the Taliban attacks the airport, it will be met with a swift response from U.S. forces in Kabul.

Meanwhile, a “coordination committee” that includes former president Hamid Karzai and Ghani’s peace negotiator Abdullah Abdullah, and that was formed to communicate with the Taliban leadership, indicated that things were proceeding toward a formal handover of power.

“We are in touch with the leaders of the respected Islamic Taliban movement,” Karzai said in a video statement. “We have positive discussions with them, we talked about necessary matters. Our ongoing cooperation is very good.”

Abdullah, in his own video, said that “our main effort at this juncture is to make sure the people of the city live in peace without any bloodshed. We know that with the new developments, people have been going through difficult times. We hope that the contacts and talks we have had about this issue and other important issues of the country turn out to be useful,” he said, noting that “we are also in touch with other leaders and tribal elders in the country.”

The committee is believed to be in direct contact with Taliban co-founder and political director Abdul Ghani Baradar, who led the militant delegation to the earlier peace negotiations and remains in Doha.

George reported from Kabul. Felicia Sonmez and John Hudson in Washington, Loveday Morris in Berlin, Rick Noack in Paris, and Jennifer Hassan in London contributed to this report.