Stunned by the Taliban's lightning advance across the country after the departure of Western troops, NATO allies have been left scrambling to evacuate their nationals as well as vulnerable Afghans.

The Taliban's return to power and chaotic scenes of people desperately seeking to get on Western military jets to flee Kabul have sparked criticism of the two-decade operation, which has cost the alliance thousands of lives and over a trillion dollars in funding.

In a meeting with her party's top brass, German Chancellor Angela Merkel admitted that developments in the country were "bitter".

She said however that once the United States decided to withdraw from Afghanistan, it was clear that Germany and other allies had to follow suit.

The decision was "ultimately made by the Americans", and "domestic political reasons" were partly to blame, said the chancellor, according to participants in the meeting.

- 'Domino effect' -

"The troop withdrawal sparked a domino effect" that culminated in the Taliban sweeping back into power, said Merkel, whose country provided the second biggest contingent of troops in the NATO operation after the US.

"For the many who have built on the progress and freedom -- especially women -- these are bitter events," she said.

The leader of her party had harsher words, calling the entire Afghanistan operation a disaster for NATO.

"It is the biggest debacle that NATO has suffered since its founding, and we're standing before an epochal change," said Armin Laschet, who is the conservative candidate to succeed Merkel as chancellor in September's elections.

The focus at the moment must be on the German military's evacuation operation from Afghanistan, he said.

"But we will talk about the causes and conclusions drawn after this rescue mission -- a no-holds-barred analysis of errors in Germany, with our allies and in the international community," he said.

Striking a more diplomatic note, Foreign Minister Heiko Maas said the failure to anticipate the speed of the Taliban's advance was a collective error.

"There is no talking this up. All of us -- the federal government, intelligence services, the international community -- misjudged the situation," Maas told a press conference in Berlin.

- 'Failure' -

Britain has also slammed the American decision to leave Afghanistan, with Defence Secretary Ben Wallace warning on Friday that the Taliban's resurgence would create a breeding ground for extremists that threatened the world.

"Of course Al-Qaeda will probably come back," he said, warning that would lead to "a security threat to us and our interests".

"I felt that that was a mistake to have done it that way, that we'll all, as an international community, probably pay the consequences of that," Wallace said of the Doha agreement signed between the United States and the Taliban.

The agreement, signed under former president Donald Trump last year, would have seen the US withdraw all its troops by May 2021 in exchange for security guarantees from the Islamist hardliners.

When Joe Biden took power earlier this year, he pushed back the deadline for the withdrawal to August 31.

Like Merkel, Wallace said the deal cut by Trump left Britain with no choice but to withdraw its troops too.

The rare criticism of the US's role by allies dovetailed with negative voices from China and Russia.

Beijing has repeatedly lambasted what it sees as the hasty withdrawal from Afghanistan as a failure of leadership, while Moscow has said the pullout meant the US had failed in its mission in Afghanistan.

For Wallace, the Taliban takeover was a "failure of the international community", as he accused the West of short-sightedness.

"If it's a failure, it's a failure of the international community to not realise that you don't fix things overnight," he said.