The United States on Saturday urged its citizens in Afghanistan to avoid traveling to the Kabul airport for now, citing "potential security threats" near its gates.

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

But conditions outside Hamid Karzai International Airport have been chaotic, with reports of people seeking to leave being beaten by Taliban fighters.

"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 alert said.

The United States hopes to evacuate 30,000 Americans and Afghan civilians through its bases in Kuwait and Qatar, and says it has already helped 13,000 people leave the country since August 14.

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 the receiving base in Qatar was overflowing.

Thousands of Afghans, already cleared to leave their country for the United States, are still waiting at the Kabul airport for available flights, while thousands of others wait outside.

US President Joe Biden has promised to help any American in Afghanistan seeking to evacuate.

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