German Foreign Minister Heiko Maas on Monday admitted NATO allies had underestimated the speed of the Taliban's advance across Afghanistan and failed to anticipate that Afghan forces were not ready to take up the fight.

"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.

The allies had not reckoned with the possibility "that the Afghan armed forces were not prepared to confront the Taliban," Maas said. "That was a misjudgement on the part of all of us."

The Taliban returned to power in Afghanistan on Sunday after the government collapsed, with president Ashraf Ghani conceding the Islamists had won and fleeing the country.

Their victory comes after US and NATO forces began withdrawing from the country in early May, almost 20 years after they arrived. 

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 end of the two-decade operation, which 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".

Armin Laschet, the conservative candidate to succeed Merkel after elections on September 26, slammed the operation as "the biggest debacle" in NATO's history.