"Ultimately, the Afghan political leadership failed to stand up to the Taliban and to achieve the peaceful solution that Afghans desperately wanted," he said.

"This failure of the Afghan leadership led to the tragedy we are witnessing today," he said, echoing US President Joe Biden, whose decision to withdraw US troops precipitated the end of the NATO mission.

Stoltenberg told reporters that the alliance was now working to ensure the safety of NATO's remaining 800 civilian personnel in Kabul and the city's airport, and of its Afghan employees.

He said that NATO member states had pledged to put on more evacuation flights at a meeting of senior envoys earlier Tuesday.

Taliban rebels overran the Afghan capital at the weekend after the Western-backed government and its US- and NATO-trained forces collapsed.

"No one anticipated the speed of the collapse of the Afghan security forces, the Afghan government and Afghan state structures," Stoltenberg said.

Staff from Western embassies have relocated to the city's airport, which is under the protection of a rear-guard of US troops deployed to cover the allied retreat.

"NATO has been working around the clock to maintain operations at the Kabul International Airport," Stoltenberg said, after a meeting of senior NATO diplomats from alliance members.

"Around 800 NATO civilian personnel have remained to provide key functions under very challenging circumstances, including air traffic control, fuel and communications."

"Let me also thank the military forces of NATO allies -- in particular Turkey, the United States and United Kingdom -- for a vital role in securing the airport."

- Gains 'hard to reverse' -

A NATO official said the alliance estimates that 1,000 Afghan employees and dependants may have to be airlifted out of Kabul.

Most would be taken to a third country while they are processed for visas to resettle in NATO member states.

Stoltenberg said NATO's senior civilian representative in Afghanistan, Ambassador Stefano Pontecorvo, would remain at the airport "to coordinate and facilitate the evacuation" of NATO staff and Afghan employees.

"The Taliban must respect and facilitate the safe departure all those who wish to leave the airport and, as well, roads and border crossings must be open," he said.

"All Afghan men, women and children deserve to live in safety and dignity."

The NATO allies deployed a powerful military force to Afghanistan in the wake of the US-led overthrow of the last Taliban government 20 years ago, and had been training local security forces.

But after the United States, the alliance's most powerful member, decided to bring an end to its mission the other members -- who had already begun to draw down their forces -- followed suit.

Stoltenberg -- who faced an emotional question from a female Afghan reporter -- insisted, however, that the time the alliance had spent trying to support Afghan society had not been wasted.

"Some of the gains will be hard to reverse -- the fact that millions of young girls and boys have got education is an achievement that is lasting," he said.

"The fact that we have more independent, stronger political voices in Afghanistan now, are gains that are not easily reversed."