Troops were seeking to build up credible and determined Afghan security forces, he added.
He said Nato's European members had to do more and not expect the US to do all the "heavy lifting".
His comments, which come ahead of a meeting with his US counterpart this week, follow the death of four US soldiers from the Nato-led International Security Assistance Force in an explosion in the province of Nangarhar, in eastern Afghanistan.
The alliance does not disclose the nationalities of its casualties but most troops in that part of the country are from the US military.
More than 60 foreign soldiers have now been killed in Afghanistan this year.
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