Washington DC – President Joe Biden has said that "chaos" was unavoidable once the United States decided to leave Afghanistan after two decades of war.

"The idea that somehow, there's a way to have gotten out without chaos ensuing, I don't know how that happens," Biden said in an interview with ABC News which aired on Wednesday when asked whether the US government made mistakes in its handling of the withdrawal from the country.

"When you had the government of Afghanistan, the leader of that government, get in a plane and taking off and going to another country; when you saw the significant collapse of the Afghan troops we had trained, up to 300,000 of them, just leaving their equipment and taking off ... That's simply what happened," Biden told ABC News.

"And so the question was, in the beginning ... do we commit to leave within the timeframe we set, do we extend it to September 1, or do we put significantly more troops in?"

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US military leaders said they were not aware of any reports or scenarios from the intelligence services that might have foreseen events unfolding so quickly.

"There was nothing that I saw that indicated a collapse of this army and this government in 11 days," the chairman of the Joint Chiefs of Staff, General Mark Milley, told a news conference on Wednesday.

"The timeframe of rapid collapse was widely estimated and ranged from weeks to months and even years following our departure," Milley said.

"There are not reports that I am aware of that predicted a security force of 300,000 would evaporate in 11 days," the general added.

US media have cited intelligence sources as saying that the Biden administration had received internal warnings of a possible very rapid collapse.

Biden said that some US troops may remain in Afghanistan past the August 31 deadline he had set for the withdrawal from the country if the evacuation of US citizens is not completed by then.

"If there are American citizens left [past August 31] we are going to stay until we get them all out," Biden said when pressed on the subject by interviewer George Stephanopoulos.

Biden insisted that the United States would try to get all US citizens and allies out of the country before the deadline. "If we don't, we'll determine at the time who is left.

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"The commitment holds to get everyone out that, in fact, we can get out and everyone who should get out," he said.

Biden said that between 10,000 and 15,000 US citizens were still in the country and an estimated 50,000-65,000 US allies, a figure which includes their families.

The United States military has flown around 2,000 people out of the Afghan capital Kabul over the last 24 hours, including 325 US citizens, according to its own figures.

In the past 24 hours, 18 C-17 transport planes departed from Kabul, Pentagon spokesperson John Kirby said on Wednesday.

He said he expects 2,000 more people will be evacuated by Thursday morning. This figure includes Afghan citizens, other civilians from NATO countries, and US citizens.

The US now has around 4,500 soldiers at Kabul airport and plans to increase the number to 6,000 soon.

The soldiers are there to ensure the security of the airport and, among other things, organize the evacuation of US citizens and former Afghan employees of the US forces.

Austin said there had been no clashes between US and Taliban forces at the airport, and that communication channels with Taliban decision-makers were open.