Speaking after the last US military flight left Kabul, Stoltenberg warned the victorious Taliban not to interfere with Afghans trying to flee the country.

After 20 years of fighting the Islamist rebels are now once again in charge of the vast majority of Afghanistan and celebrating victory over the collapsed Western-backed government.

But Stoltenberg insisted all was not lost for the allies, as their intervention had at least prevented international terror groups from launching attacks from Afghanistan on Western targets.

Now, however, he stressed the need for Kabul's new leaders to work with the international community to reopen their airport, allow Afghans who worked with the allies safe passage and to keep a lid on extremist groups.

"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 said.

"We will not forget them."

His remarks echoed those made by German Chancellor Angela Merkel, who warned earlier that the airport "is of existential importance for Afghanistan because without it no medical or humanitarian aid can get there either".

The Taliban are in talks with Turkey and Qatar to take a role in running the airport, the scene in recent weeks of an extraordinary exodus of desperate refugees and US and allied troops.

But Afghans who worked with the US or NATO members are nervous of crossing Taliban checkpoints to reach the facility, which US officials say is in bad shape.

- 'Hard questions' -

Senior European officials have suggested that British or EU civilian experts could help keep the airport running, but it is not clear whether the Taliban are willing to accept 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.

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," he promised.

"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.

"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."