There were at least two explosions near Kabul's airport amid a huge and chaotic evacuation effort from Afghanistan on Thursday, the Pentagon said, with civilians and U.S. service members among the casualties of what was described as a "complex attack."

U.S. officials said they were concerned that further attacks could occur at the airport following the twin blasts, which a Taliban official said killed at least 13 people, including children, and wounded many Taliban guards.

A U.S. official, citing initial information, told Reuters as many as five U.S. military personnel may have been hurt, including at least one seriously. U.S. officials have said there are about 5,200 American troops providing security at the Kabul airport.

The attacks came after the United States and allies urged Afghans to leave the area because of a threat by Islamic State militants.

A source familiar with U.S. congressional briefings said U.S. officials strongly believe that the Afghan affiliate of Islamic State, known as Islamic State Khorasan (ISIS-K), after an old name for the region, was responsible. ISIS-K is opposed by the United States and the Taliban.

Pentagon spokesman John Kirby said one blast occurred near the airport's Abbey Gate and the other close to the nearby Baron Hotel. Two U.S. officials said at least one of the explosions appeared to be from a suicide bombing.

"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," Kirby said on twitter. "We can also confirm at least one other explosion at or near the Baron Hotel, a short distance from Abbey Gate."

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The U.S. Embassy in Kabul described "a large explosion" and said there had been reports of gunfire.

A source who was in touch with a witness by text message quoted that witness as saying there appeared to have been two separate but simultaneous attacks, one by a suicide bomber near buses lined up outside Abbey Gate, where the blast was followed by small arms fire.

The second occurred at Baron Gate, named after the nearby Baron Hotel. The source, speaking on condition of anonymity, quoted the witness as saying that children were among the casualties.

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The attacks came after the United States and allies urged Afghans to leave the area because of a threat by Islamic State.

A massive airlift of foreign nationals and their families as well as some Afghans has been under way since the day before Taliban forces captured Kabul on Aug. 15, capping a swift advance across the country as U.S. and allied troops withdrew.

The United States has been racing to carry out the airlift before its military is set to fully withdraw from the country by Aug. 31. There was no indication from the White House that Biden plans to change the Aug. 31 withdrawal target as a result of the attacks, a source familiar with the situation told Reuters.

Biden was in a meeting with security officials about the situation in Afghanistan, where the United States is in the final steps of ending its 20-year war, when the explosion was first reported, according to a person familiar with the matter.

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Biden, Secretary of State Tony Blinken, Defense Secretary Lloyd Austin, Joint Chiefs of Staff Chairman General Mark Milley and Vice President Kamala Harris monitored events via video links.

In an alert issued on Wednesday, the U.S. Embassy in Kabul had advised citizens to avoid traveling to the airport and said those already at the gates should leave immediately, citing unspecified "security threats."

A Western diplomat in Kabul said that areas outside the airport gates had been "incredibly crowded" again despite the warnings.

The United States and its allies have mounted one of the biggest air evacuations in history, bringing out about 95,700 people, including 13,400 on Wednesday, the White House said on Thursday.

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