The Islamic State group claimed responsibility for the killings on its Amaq news channel

Two suicide bombers and gunmen attacked crowds of Afghans flocking to Kabul's airport on Thursday, transforming a scene of desperation into one of horror in the waning days of an airlift for those fleeing the Taliban takeover. The attacks killed at least 77 Afghans and 13 U.S. troops, Afghan and U.S. officials said.

The U.S. general overseeing the evacuation vowed the United States would “go after” the perpetrators of the bombings, and warned that more such attacks are expected.

“We are working very hard right now to determine attribution, to determine who is associated with this cowardly attack. And we’re prepared to take action against them,” Gen. Frank McKenzie, head of U.S. Central Command, told Pentagon reporters in a briefing. “Twenty-four-seven. We are looking for them.”

Shortly after Gen. McKenzie spoke, the Islamic State group claimed responsibility for the killings on its Amaq news channel. Gen. McKenzie said the attacks would not stop the United States from evacuating Americans and others, and flights out were continuing. He said there was a large amount of security at the airport, and alternate routes were being used to get evacuees in. In addition to the many Afghans, the State Department estimated there were as many as 1,000 Americans in Afghanistan who may want help getting out.

U.S. officials said 11 Marines and one Navy medic were among those who died. Gen. McKenzie said another 15 service members were wounded. Officials warned the toll could grow. More than 140 Afghans were wounded, an Afghan official said.

One of the bombers struck people standing knee-deep in a wastewater canal under the sweltering sun, throwing bodies into the fetid water. Those who moments earlier had hoped to get on flights out could be seen carrying the wounded to ambulances in a daze, their own clothes darkened with blood.

The IS affiliate in Afghanistan is far more radical than the Taliban, who recently took control of the country in a lightning blitz and condemned the attack.

Western officials had warned of a major attack, urging people to leave the airport, but that advice went largely unheeded by Afghans desperate to escape the country in the last few days of an American-led evacuation before the U.S. officially ends its 20-year presence on August 31.