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 US service members among the casualties of what was described as a “complex attack.”

A Taliban official said at least 13 people were killed in the blast, including children, and many Taliban guards were wounded.

Pentagon spokesman John Kirby said one blast occurred near the airport's Abbey Gate and the other close to the nearby Baron Hotel. Two US 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."

A US official told Reuters as many as three US service members were among those hurt, and that US casualty numbers were expected to increase, according to initial information. At least one of the US personnel was seriously hurt, an official said.

The US 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.

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 August 15, capping a swift advance across the country as US and allied troops withdrew.

President Joe Biden has been briefed on the explosion, according to a White House official. 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.