He said 2,000 suicide bombers were ready - about 40 per cent of the total suicide force - and that numbers were so high it was sometimes hard to find enough explosives and targets.
More than 4,000 people died last year, a quarter of them civilians, as the resurgent Taliban fought back with what Nato generals said was surprising ferocity.
Calling the fighters virulent and tough, Richard Boucher, US assistant secretary of state for south and central Asia, said on Friday that the spring would be bloody and dangerous after the winter lull in fighting.
"I think we will face a strong offensive and will have a difficult and dangerous and bloody spring," he said.
"But we are also better set up to deal with it."
While urging Nato allies to do more, the United States has extended troop tours of duty - effectively boosting its forces by 2,500 - and announced $10.6bn in extra aid for Afghan security forces and reconstruction.