Kabul (AFP) - Tens of thousands of Afghans were racing Sunday to flee their country as the United States warned of security threats at Kabul's chaotic airport and the European Union said it was "impossible" to evacuate everyone at risk from the Taliban.

In the week since the hardline Islamist militants took back power in Afghanistan, the Taliban have vowed a softer version of their brutal rule from 1996-2001, and taken steps towards forming a government.

But terrified Afghans continue to try and flee, deepening a tragedy at Kabul airport where the United States and its allies have been unable to cope with the huge numbers of people trying to get on evacuation flights.

Britain's Sky News on Saturday aired footage of at least three bodies covered in white tarpaulin outside the airport. It was not clear how they had died.

Sky reporter Stuart Ramsay, who was at the airport, called the deaths "inevitable" and said people were being "crushed", while others were "dehydrated and terrified".

The footage was the latest image of utter despair, after video of a baby being lifted over a wall at the airport and horror scenes of people hanging onto departing planes.

Families hoping for a miracle crowded between the barbed-wire that surrounds an unofficial no man's land separating the Taliban from US troops. Roads to the airport were choked with traffic. 

President Joe Biden has described the situation as "one of the largest, most difficult airlifts in history".

Circumstances were further complicated on Saturday when the US government warned its citizens to stay away from the airport.

No specific reason was given, but a White House official later said Biden had been briefed on "counter terrorism" threats, including the Islamic State group.

'Impossible' deadline

The United States, which has thousands of troops trying to secure the airport, has set a deadline to complete the evacuations by August 31.

But there are up to 15,000 Americans and 50,000 to 60,000 Afghan allies who need to be evacuated, according to the Biden administration.

There are countless others, including journalists, who fear repression under the Taliban and are also trying to flee.

"We know that we're fighting against both time and space," Pentagon spokesman John Kirby conceded, while others gave a bleaker assessment.

"They want to evacuate 60,000 people between now and the end of this month. It's mathematically impossible," European Union foreign policy chief Josep Borrell told AFP.

Borrell added that "we have complained" to the Americans that their airport security was overly strict and hampering attempts by Afghans who worked for the Europeans to enter. 

Biden has said the deadline could be extended for the airlifts. Borrell said that as far as he knew that decision had not yet been made. 

On Saturday the Pentagon said 17,000 people had been taken out since the operation began on August 14, including 2,500 Americans.

Thousands more have left on other foreign military flights.

Taliban government

The Taliban have been publicly content to allow the US military oversee the airlift, while focusing on how they will run the country once the foreign forces leave.

Taliban co-founder Mullah Abdul Ghani Baradar flew into Kabul on Saturday, and officials said they were mapping out an "inclusive government".

A senior Taliban official told AFP that Baradar would meet jihadi leaders, elders and politicians in coming days.

The gathering in Kabul included leaders of the Haqqani network, a US-designated terrorist organisation with million-dollar bounties on its leadership.

'Imbecilic' withdrawal

The Taliban swept into Kabul last week, ending two decades of war, stunning the world as government forces surrendered en masse and raising questions about Washington's ability to support its allies.

Former UK prime minister Tony Blair, who in 2001 took Britain into war in Afghanistan alongside the United States, on Saturday criticised the US motives for the withdrawal as "imbecilic".

"The abandonment of Afghanistan and its people is tragic, dangerous, unnecessary, not in their interests and not in ours," Blair wrote in a wide-ranging article.

"We didn't need to do it. We chose to do it.