The West had "underestimated" the Afghan conflict, NATO Secretary General Anders Fogh Rasmussen said in a newspaper interview published Monday, a day ahead of a major conference in Kabul, DPA reported.
"It is undeniable that, at the beginning, the international community underestimated the scale of this challenge," he told the German daily Hamburger Abendblatt.
On Tuesday, the Afghan capital Kabul will host a key conference of some 60 envoys from NATO states, including US Secretary of State Hillary Clinton and UN chief Ban Ki-moon, where delegates will seek ways to hand more power to the Afghan governments despite record violence.
"It has become painfully clear, after nine years of this international engagement, that the price that we have to pay is much higher than expected - particularly in view of the number of international and Afghan soldiers killed.
According to the independent agency iCasualties.org, 1,946 international troops have been killed in Afghanistan since the US-led invasion of 2001.
Rasmussen said that a future offensive to beat back the Taliban in the south of the country would undoubtedly lead to more casualties.
"Regrettably there will be more victims. But these military campaigns are of enormous political importance. They will contribute to the political and military weakening of the Taliban," he said.
The NATO chief said that it was his goal to hand over security responsibility to the Afghan forces, but continue to support the national troops over the long term.
US President Barack Obama said in December that Washington plans to begin its troop pullout from Afghanistan in July 2011, a goal endangered by the spike in violence this year.