"It's essential to keep the airport open, both to enable humanitarian aid to the Afghan people and also to make sure that we can continue to get people out -- those who wished to, but were not able to be part of the military evacuation," he told AFP in an interview.

"We will not forget them."

The last US military flight departed from Kabul's airport late Monday after a rushed airlift to rescue more than 123,000 allied personnel and Afghans who worked alongside them during the conflict.

But as the Taliban celebrated their victory, the NATO alliance is left to analyse what went wrong in its most important military mission since the Cold War.

And Stoltenberg swore the allies would maintain diplomatic pressure on the Taliban to allow the remaining Afghans, and their families, who worked to help the Western effort and now feel at risk, to leave the country.

He praised Turkey, a NATO member, for offering to take a role in running the airport as the Taliban try to get it open, and thanked roughly 800 NATO civilian staff for their help in managing the airlift.

"We will continue to work with NATO allies, with other countries to help people to leave. Taliban has clearly stated that people will be allowed to leave, we will judge Taliban not on what they say, but by what they do," he said.

"And we will use our political, diplomatic, economic leverage to ensure that people are able to leave. This is important because the NATO allies have been there for so many years."

Looking forward, Stoltenberg said the 30-member western alliance would have to look carefully at what went wrong in their mission to build an Afghan government and military capable of holding off the Taliban advance.

"These are among the hard questions we have to ask, when we now will have a process where we're going to assess, analyse, and have our lessons learned process in NATO," he told AFP.

"Because we need to understand better, both what went wrong, but also to analyse achievements we made in Afghanistan, not least when it comes to fighting terrorism."