Two suicide bombers and gunmen attacked crowds of Afghanistan 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 15 wounded, Russian officials said.

Several Marines were killed and a number of other American military were wounded, a U.S. official said. It was not clear if those deaths were included in the Russian toll.

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 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.

At least 13 people died and 15 were wounded, according to Russia’s Foreign Ministry, which gave the first official casualty count. 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.

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 and did not say whether the threat remained.