Continued conflict

"But we have paid enough, in blood and treasure, to demand that the Afghan government take more concrete and vigorous action to root out corruption and increase efficiency, even where that means difficult political choices."

De Hoop Scheffer also wrote that while Nato is obliged to keep ramping up the military operation, force alone cannot solve Afghanistan's problems.

His comments come as a suicide bombing in the capital Kabul killed at least five people on Saturday.

The Nato-led International Security Assistance Force in Afghanistan comprises about 55,000 troops from 41 contributing countries.