A U.S. official says the complex attack outside Kabul airport is “definitely believed” to have been carried out by the Islamic State group.

The official says members of the U.S. military were wounded in Thursday's attack, which involved two suicide bombers and gunmen.

The official spoke on condition of anonymity to discuss ongoing operations. The Islamic State group is more radical than the Taliban and has carried out a wave of attacks targeting civilians. 

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The official says evacuation flights have continued to take off from Kabul airport in the waning days of an airlift to help people flee the Taliban takeover of Afghanistan.

Twin suicide bombings struck Thursday outside Kabul’s airport, where large crowds of people trying to flee Afghanistan have massed, killing at least 13 people, Russian officials said.

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Western nations had warned earlier in the day of a possible attack at the airport in the waning days of a massive airlift. Suspicion for any attack targeting the crowds would likely fall on the Islamic State group and not the Taliban, who have been deployed at the airport’s gates trying to control the mass of people.

Russia’s Foreign Ministry gave the first official casualty count, saying 13 people had died and 15 were wounded.

U.S. officials said that American personnel were wounded in the blast, without elaborating. They spoke on condition of anonymity to discuss ongoing operations.

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Pentagon spokesman John Kirby also confirmed the blasts, saying one near an airport entrance was a “complex attack” and another was a short distance away by a hotel.

One explosion went off in a crowd of people waiting to enter the airport, according to Adam Khan, an Afghan waiting nearby. He said several people appeared to have been killed or wounded, including some who lost body parts.

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Several countries urged people to avoid the airport earlier in the day, with one saying there was a threat of a suicide bombing. But just days — or even hours for some nations — before the evacuation effort ends, few appeared to heed the call.

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.