The warning comes a day after a top US diplomat warned Afghanistan was in for a bloody and dangerous spring after the bloodiest year since the hardline Islamist Taliban was ousted by USled forces in 2001.

"The Taliban will intensify their guerrilla and suicide strikes this summer, " Mulla Hayat Khan told reporters from a secret location. "This will be the bloodiest year for foreign troops."

He said 2,000 suicide bombers were ready - about 40% of the total suicide force - adding numbers were so high it was sometimes hard to find enough explosives and targets.

"Our war preparations have been completed to a large extent and we're waiting for summer to set in, " Khan said.

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 guerrillas virulent and tough, US assistant secretary of state for south and central Asia, Richard Boucher, warned on Friday that the spring would be bloody and dangerous after the traditional winter lull in fighting.

"I think we will face a strong offensive and will have a difficult, dangerous and bloody spring, " he told BBC radio as NATO foreign ministers discussed Afghanistan's future at a summit in Brussels called by Washington.

"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 (Euro8.2bn) in extra aid for Afghan security forces and reconstruction.