In the final days of the 20-year U.S. presence in Afghanistan, the bombing caused the second-deadliest American toll of the war. The blasts struck in the middle of a dense crowd of families at the airport gates who were desperately hoping to make one of the last evacuation flights out. Gunfire was reported in the aftermath of the explosions.

The American toll was confirmed by Gen. Kenneth F. McKenzie Jr., head of the United States Central Command. He said that 15 more American troops were wounded, and warned Thursday’s attack may not be the last one.

“We have other active threats against the airfield,” he told reporters during a news conference in Washington.

The American troops, mostly Marines, were part of the deployment of 5,800 sent by President Biden to help evacuate Americans and Afghan allies from the country after the fall of Kabul to the Taliban.

Estimates of the total dead and wounded differed, and were rising quickly as different hospitals and officials reported in.

One Afghan health official said at least 60 people were confirmed dead and at least 140 wounded. Another health official said at least 40 were dead and 120 wounded. Both officials spoke on the condition of anonymity because the Taliban told them not to brief the press, they said.

The Taliban spokesman, Zabihullah Mujahid, condemned the attack, and said that at least 13 civilians had been killed and 60 wounded.

In one part of one hospital alone, a New York Times journalist saw dozens of severely wounded or killed people.

The night before, a senior U.S. official warned of a “specific” and “credible” threat at the airport by an affiliate of the Islamic State, the Islamic State Khorasan, or ISIS-K, and Western governments began urging people to leave the area.

Even with such a specific warning, military officials said, it would be very difficult to pick out a suicide bomber with a concealed explosive vest in a huge throng of people, like that at the airport.

Defense Secretary Lloyd Austin said of the American service members, many of them Marines, that “terrorists took their lives at the very moment these troops were trying to save the lives of others.”

In a statement Thursday, Mr. Austin said that “we will not be dissuaded from the task at hand,” seeming to indicate that evacuations from Kabul airport would continue in the last four days before the Aug. 31 deadline. “To do anything less — especially now — would dishonor the purpose and sacrifice these men and women have rendered our country and the people of Afghanistan.”

Since the Taliban takeover earlier this month, thousands of Afghan civilians and foreign citizens have gathered at the gates of the airport, which has a military and civilian side, desperate to be airlifted out of the country. At times, the area has descended into chaos as people scrambled toward evacuation flights.

Two U.S. military officials said evacuation flights were continuing, though it was not clear whether any gates at the airport were open.

“We can confirm that the explosion at the Abbey Gate was the result of a complex attack that resulted in a number of US & civilian casualties,” John Kirby, the Pentagon press secretary, said in a post on Twitter. “We can also confirm at least one other explosion at or near the Baron Hotel, a short distance from Abbey Gate.”

The Abbey Gate is a main entryway to the international airport. The U.S. Embassy in Kabul warned citizens to avoid traveling to the airport and avoid airport gates, and urged Americans who were at the Abbey Gate, East Gate or North Gate entrances to leave immediately.

U.S. military officials at the airport said that an attack, given the speed and confusion surrounding the entire evacuation, was never a matter of if, but when. The U.S. Marines guarding Abbey Gate had been briefed on the potential of a suicide bomber striking near their position, but continued processing those trying to gain entry.

One Afghan, Barat, who had traveled to the airport with his cousin to show documents to foreign soldiers, said he was about 30 feet away from one of the blasts.

“The crowd was packed and people were pushing,” he said. “I tripped — and that’s when the explosion happened. I think four or five soldiers were hit.”

Then chaos.

“We fell to the ground and the foreign soldiers started shooting,” Barat said. “There were bodies everywhere, people were running.”

Fahim, a shopkeeper from Kunduz Province, came to Kabul two weeks ago in an attempt to leave the country, and was outside the airport when he witnessed what he described as “two big explosions” nearby. “People were fleeing and the Taliban forced us to leave the area,” he said.

“Americans were firing to disperse people,” Fahim said.

Elsewhere in the city, sporadic gunfire and alarms could be heard from the airport.