WASHINGTON — There were at least two explosions near Kabul's airport amid a huge and chaotic evacuation effort from Afghanistan on Thursday (Aug 26), the Pentagon said, with civilians and US service members among the casualties of what was described as a "complex attack".

A source familiar with US congressional briefings said US 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.

A Taliban official said at least 13 people were killed in the blast, including children, and many Taliban guards were wounded. A US official, citing initial information, told Reuters as many as five US military personnel may have been hurt, including at least one seriously.

A surgical hospital run by an Italian charity said it was treating more than 60 wounded. The attacks came after the United States and allies urged Afghans to leave the area because of a threat by Islamic State militants.

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," Mr 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."

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 Aug 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. Mr 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.

The United States has been racing to carry out the airlift before its military is set to fully withdraw from the country on Aug 31.

In an alert issued on Wednesday, the US Embassy in Kabul had advised citizens to avoid travelling 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. REUTERS