At least 13 civilians, including children, were reportedly killed in explosions at Kabul airport in Afghanistan.
Two separate bombings are said to have taken place, at least one of which was caused by a suicide bomber.
US defense spokesman John Kirby confirmed that the explosion occurred Thursday afternoon.
Mr Kirby said: “We can confirm an explosion outside Kabul airport. The casualties are unclear at this time. We will provide further details when we can.”
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Three US soldiers and Taliban guards were also reportedly injured in the attack.
It was not immediately clear whether British troops were among the casualties and evacuation planes continued to take off from Kabul within minutes of the explosion.
Seconds after the explosion, gunmen reportedly opened fire as the airport was attacked by Islamic State fighters desperate to deliver a final blow to the mission.
The “complex attack” came hours after the Taliban captured two suspected ISIS fighters.
They are believed to have been caught while probing the airport for security loopholes.
The attack took place in the canal where people wait to see if they can get a flight to leave the country, as the evacuation deadline draws near.
The explosion took place near the US canal area, where civilians were reportedly standing near the bomber at around 6 p.m. local time (3 p.m. BST).
It is suspected that a second explosion could have been set off shortly after the first, indicating that several suicide bombers are targeting the evacuation effort.
People outside the airport were told to leave the area immediately as concerns about violence from an ISIS-affiliated group mounted.
The airport has become the evacuation center for expatriates and Afghans and their families who have helped foreign troops, since the Taliban took control of the country last week.
Footage from the scene shows people, including children, walking around in shock. People can be seen with visible wounds and blood.
Australia, the United States and the United Kingdom had all issued alerts to their citizens on Thursday morning, warning them of a potential attack.
The Defense Ministry tweeted: “We are urgently working to establish what happened in Kabul and its impact on the ongoing evacuation effort.
“Our main concern remains the safety of our staff, British citizens and Afghan citizens.
Earlier today, Prime Minister Boris Johnson said the “overwhelming majority” of those eligible for evacuation from Afghanistan had now been removed.
Conservative MP Alicia Kearns, a member of the Foreign Affairs and National Security Strategy committees, said there were “many injured” in an attack on the Hotel Baron, where Britons and Afghans eligible for repatriation were being processed.
“A bomb or a gunshot attack on the north door of the Hotel Baron. I’m worried it will devastate the evacuation – so many injured. My heart is with all those injured and killed,” a- she declared.
Official figures suggest that more than 100,000 people have now been evacuated from Afghanistan since the Taliban took control of the nation.
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