The United States on Sunday enlisted several major airlines in its frantic evacuation of tens of thousands of Afghans, Americans and other foreigners from Kabul following its fall to Taliban extremists.

Secretary of Defense Lloyd Austin activated the rarely-used Civil Reserve Air Fleet (CRAF) to aid the onward movement of people arriving at US bases in the Middle East, the Pentagon said.

"We're going to try our very best to get everybody, every American citizen who wants to get out, out," Austin said in an ABC interview on the evacuation mission, adding that the same applied to America's Afghan allies.

Eighteen civilian craft, from American Airlines, Atlas, Delta, Omni, Hawaiian and United, will aid dozens of military cargo transports involved in the evacuation, the statement said.

Rather than going in and out of Kabul, the planes will transport people from US bases in Qatar, Bahrain and the United Arab Emirates to European countries and, for many, onward to the United States.

With thousands of troops trying to secure the Afghan capital's airport, Washington has set a deadline to complete one of the largest evacuation missions the Pentagon has ever conducted by August 31.

But Austin didn't rule out asking the president to extend the deadline.

"We're going to continue to assess the situation. And again, work as hard as we can to get as many people out as possible. And as we approach that deadline, we'll make a recommendation to the president," he told ABC.

Up to 15,000 Americans need to be removed from Afghanistan, according to President Joe Biden, who says the administration wants to get at least 50,000 Afghan allies and their family members out of the country.

Widely criticized over the chaotic exit after a sudden Taliban victory, Biden has warned that the frantic effort to fly Americans, other foreigners and Afghan allies out of Taliban-occupied Kabul is dangerous.

There have been reports of Taliban militants intimidating and beating people trying to reach the airport but Austin said the militants had largely been letting American passport holders pass through safely.

The situation was further complicated on Saturday when the US government warned its citizens to stay away from the airport because of "security threats."

The Pentagon said Saturday that 17,000 people had so far been taken out since the operation began on August 14, with many flown first to Qatar or Kuwait. The total included 2,500 Americans.

US National Security Advisor Jake Sullivan told CNN Sunday that the US military and its partners had flown out a further 7,900 people in the last 24 hours.

"In terms of what we'll be able to accomplish going forward... we can't place a specific figure on exactly what we'll be able to do, but I'll just tell you that we're going to try to exceed expectations, and do as much as we can, and take care of as many people as we can, for as long as we can," Austin told ABC.

The CRAF has only been activated twice -- to fly troops for the 1990-91 Gulf War and again in 2002-2003 for the Iraq invasion.

US orders American airlines to aid Afghan evacuation

The United States on Sunday enlisted several major airlines in its frantic evacuation of tens of thousands of Afghans, Americans and other foreigners from Kabul following its fall to Taliban extremists.

Secretary of Defense Lloyd Austin activated the rarely-used Civil Reserve Air Fleet (CRAF) to aid the onward movement of people arriving at US bases in the Middle East, the Pentagon said.

"We're going to try our very best to get everybody, every American citizen who wants to get out, out," Austin said in an ABC interview on the evacuation mission, adding that the same applied to America's Afghan allies.

Eighteen civilian craft, from American Airlines, Atlas, Delta, Omni, Hawaiian and United, will aid dozens of military cargo transports involved in the evacuation, the statement said.

Rather than going in and out of Kabul, the planes will transport people from US bases in Qatar, Bahrain and the United Arab Emirates to European countries and, for many, onward to the United States.