An explosion erupted outside the Abbey gate at the Hamid Karzai International Airport in Kabul Thursday morning, resulting in multiple American casualties, Pentagon spokesman John Kirby confirmed.

We can confirm that the explosion at the Abbey Gate was the result of a complex attack that resulted in a number of US & civilian casualties. We can also confirm at least one other explosion at or near the Baron Hotel, a short distance from Abbey Gate. We will continue to update.

— John Kirby (@PentagonPresSec) August 26, 2021

At least three U.S. servicemen were injured in the blast, Lucas Tomlinson of Fox News reported.

The assault was reportedly perpetrated by a suicide bomber armed with an improvised explosive device. The suicide bomber was affiliated with ISIS, a U.S. official told Politico.

A firefight followed the detonation, according to Fox News’s Jennifer Griffin.

Shortly after the first blast, a second explosion occurred outside the Barron Hotel, where Afghans and Americans have gathered while awaiting evacuation. It’s unclear whether Americans were injured in that blast.

In recent days, reports have circulated that the terror threat risk calculation has been significantly elevated for the Kabul airport, from which the U.S and allied forces are conducting a massive evacuation of civilians out of the war-torn country to safe haven nations in Europe and elsewhere.

The explosion comes hours after the British military warned that the airport may be the target of an “imminent attack” designed to disrupt the airlift.

An anonymous senior U.S. official confirmed to Fox News Wednesday the high risk assessment of Britain's armed forces minister James Heappey, adding that the odds of an ISIS-K or similar group attacking using a vehicle bomb or suicide bomber has been “more likely than unlikely” for over the last 24 hours.

The U.S. embassy in Kabul issued an alert Wednesday evening local time warning Americans to avoid the airport “[b]ecause of security threats outside the gates of Kabul airport.”