Two armed Afghan men were killed by US forces occupying Hamid Karzai International Airport as thousands sought to board flights to flee the takeover of the country by the Taliban.

"There are no flights coming or going, military or civilian, and this is because of large crowds that are still on the tarmac," Pentagon spokesman John Kirby told reporters.

"US military forces are on the scene working alongside Turkish and other international troops to clear the area of people. We do not know how long this will take," Kirby said.

He said it could be hours before flights restart.

He pinned the blame for the chaos on the Afghan leadership, much of which has already fled the country.

"You can resource, you can train, you can support, you can advise, you can assist. You cannot buy will, you cannot purchase leadership, and leadership was missing," he said.

- More US soldiers -

Kirby said a third battalion of US soldiers would be deployed to the airport as Washington seeks to evacuate embassy personnel and as many as 30,000 Afghans and their families, people who worked with US forces, many as translators, and who are seeking refuge in the United States.

That could take the number of US troops in the country back up to around 6,000 in the coming days.

"Our focus is on making sure that we can get the security at the airport restored, we can get flights going in and out again, and we can start to process people to get them on flights," Kirby said.

The US has already carried out dozens of flights of its giant C-17 cargo jets, which can carry several hundred people at a time.

But on Monday videos taken from the airport showed hundreds of Afghans flooding onto the runways and trying to impede the takeoff of one of the US transports.

Local reports said several were killed, some possibly falling from the plane after it took off.

Kirby did not confirm the deaths but said of the videos, "that's why we want to restore a safe and secure environment so that nobody else can get hurt."

He confirmed that Central Command General Kenneth McKenzie -- the top US military officer in the Middle East -- warned the Taliban in Doha on Sunday not to interfere with US evacuation operations.

McKenzie told them "that any attack on our operations or people will be met with a forceful response," Kirby said.

"We have not seen an attack occur," he added.