NATO Secretary-General Anders Fogh Rasmussen was speaking before a meeting with alliance defence ministers in Bratislava on a new approach against the widening Taliban insurgency.
President Barack Obama is still considering a call from the top US and NATO commander in Afghanistan, General Stanley McChrystal, for tens of thousands more soldiers.
We all have to achieve more in training and equipping the Afghan security forces, Rasmussen told a security conference before the ministers meeting in the Slovak capital, which is not expected to announce decisions on troop levels.
We need other international actors to redouble their efforts to help with reconstruction and development. We have to do more today if we want to be able to do less tomorrow.
Rasmussen said NATO, which wants Afghan security forces eventually to take over defence tasks, had a mission, which was vital for the security of the region and of NATO states.
I am well aware that there is an increasing number of people who are asking if the cost of our engagement in Afghanistan is too high, he said, referring to waning public support for the NATO effort in many countries.
To these people, I want to say very clearly and unambiguously that the cost of inactivity would be far higher.
Im well aware that there is an increase in the number of people ... who are asking if the costs of our engagement in Afghanistan is too high, he told a conference of defence experts. The costs of inaction would be far higher.
Leaving Afghanistan behind would once again turn the country into a training ground for Al-Qaeda. The pressure on nuclear-armed Pakistan would be tremendous, insecurity would spread throughout Central Asia and it would only be a matter a time until we in Europe would feel the consequences, he said.
In a new book, Canadas former top general, Rick Hillier, warned: Afghanistan has revealed that NATO has reached the stage where it is a corpse, decomposing and in need of lifesaving or the alliance will be done.
It was crystal clear from the start that there was no strategy for the mission in Afghanistan, he wrote.
We need to ask and to insist on much more from the incoming Afghan government when it comes to fighting corruption, improving government, said NATO spokesman James Appathurai.
When the Afghan government is trusted by Afghan people to provide effective services that will suck the oxygen away from the insurgency, that is an essential element in the fight against the insurgency, he said.
We all have to invest more in training and equipping the Afghan security forces and we need other international actors to beef up their efforts to help with reconstruction and development, Rasmussen said.
It is in fact a very simple calculation: we have to do more today in order to be able to do less tomorrow, he added.