KABUL, Afghanistan (AP) — Two suicide bombers and gunmen attacked crowds of Afghans flocking to Kabul’s airport Thursday, transforming a scene of desperation into one of horror in the waning days of an airlift for those fleeing the Taliban takeover. At least 13 people were killed and dozens wounded, Russian officials and aid workers said.

At least 12 U.S. service members were killed in the Afghanistan bombings, including 11 Marines and one Navy medic. 15 military members were injured Marine General Kenneth McKenzie Jr said.

Officials warned, however, that the numbers may grow.

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.

A second U.S. official said the complex attack was believed to have been carried out by the Islamic State group. 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 Aug. 31.

Russia’s Foreign Ministry gave the first official casualty count, but the numbers continued to grow. Emergency, an Italian charity that operates hospitals in Afghanistan, said it had received 60 patients wounded in the airport attack, in addition to six who were dead when they arrived.

Pentagon spokesman John Kirby also confirmed the blasts and said there were casualties, including among members of the military, but gave no figure. He said one explosion was near an airport entrance and another was a short distance away by a hotel.

One official, who spoke on condition of anonymity to discuss ongoing military operations, said several Marines were killed. It wasn’t clear if other U.S. military troops were among the dead. American officials have said that information is still coming in and they are trying to determine exact numbers of casualties.

Even as the area was hit, evacuation flights continued to take off from Kabul airport.

Adam Khan was waiting nearby when he saw the first explosion outside what’s known as the Abbey gate. He said several people appeared to have been killed or wounded, including some who were maimed.

The second blast was at or near Baron Hotel, where many people, including Afghans, Britons and Americans, were told to gather in recent days before heading to the airport for evacuation.

A former Royal Marine who runs an animal shelter in Afghanistan says he and his staff were caught up in the aftermath of the blast near the airport.

“All of a sudden we heard gunshots and our vehicle was targeted, had our driver not turned around he would have been shot in the head by a man with an AK-47,” Paul “Pen” Farthing told Britain’s Press Association news agency.

Farthing is trying to get staff of his Nowzad charity out of Afghanistan, along with the group’s rescued animals.

He is among thousands trying to flee. Over the last week, the airport has been the scene of some of the most searing images of the chaotic end of America’s longest war and the Taliban’s takeover, as flight after flight took off carrying those who fear a return to the militants’ brutal rule. When the Taliban were last in power, they confined women largely to their home and widely imposed draconian restrictions.

Already, some countries have ended their evacuations and begun to withdraw their soldiers and diplomats, signaling the beginning of the end of one of history’s largest airlifts. The Taliban have insisted foreign troops must be out by America’s self-imposed deadline of Aug. 31 — and the evacuations must end then, too.

In Washington, U.S. President Joe Biden spent much of the morning in the secure White House Situation Room where he was briefed on the explosions and conferred with his national security team and commanders on the ground in Kabul.

Overnight, warnings emerged from Western capitals about a threat from IS, which has seen its ranks boosted by the Taliban’s freeing of prisoners during its advance through Afghanistan.

Shortly before the attack, the acting U.S. ambassador to Kabul, Ross Wilson, said the security threat at the Kabul airport overnight was “clearly regarded as credible, as imminent, as compelling.” But in an interview with ABC News, he would not give details.