The mission managed to stop Al-Qaeda repeating its September 11, 2001 attacks on the United States, but "everything else that has followed has not been as successful and has not been achieved in the way that we had planned", Merkel told journalists.

The rapid return of the Taliban to power was "particularly dramatic and terrible", said the chancellor.

"It is terrible for the millions of Afghans who had worked for a freer society and who, with the support of the Western community, have focused on democracy, on education, on women's rights," she said.

It was also devastating for the loved ones of soldiers who paid with their lives in the NATO operation, she said.

Lessons must be drawn from the two-decade long operation, she said, adding that "you also have to set smaller goals, I think, in such missions".

The leader of Merkel's party, Armin Laschet, had earlier Monday called the Afghanistan operation the "biggest debacle" in the alliance's history.