US President George W Bush has called on other Nato members to step up their battle against Afghanistan's Taleban.
A few nations provide the bulk of the Nato fighting force
Mr Bush said member nations had to listen to commanders when they asked for more troops and be more flexible in how they allowed their forces to fight.
He also vowed to launch a new offensive against the Taleban, saying: "We will not give in."
Taleban forces have traditionally launched spring offensives after lulls in fighting over the winter.
The governor of southern Helmand province said this week up to 700 insurgents had crossed from Pakistan and were preparing to fight Nato.
Mr Bush, in a speech at the American Enterprise Institute in Washington, said: "When our commanders on the ground say to our respective countries, 'we need additional help', our Nato countries must provide it."
He said that Nato had to hold true to its founding principle - "an attack on one is an attack on all".
"Allies must lift restrictions on the forces they do provide so Nato commanders have the flexibility they need to defeat the enemy wherever the enemy may make its stand," he said.
On Thursday, the US said a redeployment from the 173rd Airborne Brigade would keep US troop levels in Afghanistan at 27,000, the highest since the invasion of 2001.
Canada, Britain, the Netherlands and the US provide the bulk of the fighting forces. The US has led calls for other member nations to do more.
Mr Bush said a top-to-bottom review of US strategy in Afghanistan had been completed.
He said he was asking Congress for $11.8bn (£6bn) over the next two years to promote the democratic future of Afghanistan.
Mr Bush said that in 2006, the number of roadside bombs in Afghanistan had doubled and suicide bombings had grown nearly fivefold.
Mr Bush said: "The snow is going to melt in the Hindu Kush mountains and when it does we can expect fierce fighting to continue.
"Our strategy is not to be on the defence but to go on the offence. This spring there's going to be a new offensive in Afghanistan and it's going to be a Nato offensive."
The president also vowed to tackle poppy cultivation, saying it was "a direct threat to the future of Afghanistan".