The warning, posted on the website of the US Embassy in Afghanistan and tweeted by the State Department in Washington, provided no detail on the nature of the threat.

But conditions outside Hamid Karzai International Airport have been chaotic amid the crush of people hoping to flee the Taliban takeover of the country.

As thousands of Americans and Afghans wait in the airport for flights or gather outside its gates, there have been "sporadic" reports, confirmed by the Pentagon, of Taliban fighters beating and harassing people trying to flee.

"Because of potential security threats outside the gates at the Kabul airport, we are advising US citizens to avoid traveling to the airport and to avoid airport gates at this time unless you receive individual instructions from a US government representative to do so," the US embassy alert said.

- 17,000 evacuated -

Underlining the threat that the White House sees in the unfolding chaos -- and likely also due to concern over a hurricane approaching the US northeast -- President Joe Biden canceled a planned trip home to Delaware Saturday.

Pentagon officials, speaking not long after the warning was issued, declined to offer details of the threat, merely saying they were continuing to process people reaching the airport gates.

"There has been no reported change to the current enemy situation in and around the airport at this time," Major General Hank Taylor said.

He said 17,000 people had been evacuated since the operation began on August 14, with many flown first to Qatar or Kuwait. The total included 2,500 Americans.

In the past 24 hours, Taylor said, six military C-17 planes and 32 charter flights had departed the Kabul airport, carrying 3,800 people.

Taylor said three flights had already reached Dulles International Airport outside Washington, adding that Afghan citizens were being sent on to the Fort Bliss army base in western Texas for processing.

The United States hopes to evacuate a total of 30,000 Americans and Afghan civilians.

On Friday, the US military sent helicopters to rescue over 150 Americans unable to reach the airport gates, an official said.

That was the first evidence that US forces were willing and able to go beyond the US-secured compound to help people seeking evacuation.

American officials earlier confirmed that evacuation operations from Afghanistan had stalled for about seven hours Friday, because a receiving base in Qatar was overflowing.

Biden has promised to help any American in Afghanistan seeking to evacuate, saying, "Any American who wants to come home, we will get you home."

But he has admitted that the presence of thousands of US soldiers at the airport does not guarantee safe passage to that vast compound.