US Defence Secretary Robert Gates has warned the future of Nato is at risk due to differences over Afghanistan and that it may become a two-tier alliance.
Mr Gates said that without more sharing of the burden of counter-insurgency in Afghanistan, the willingness of those engaged in combat would disappear.
Most of the fighting in the volatile south of the country is being done by the US, UK, Canada and the Netherlands.
His comments came as Germany announced it would send 200 troops to the north.
German Defence Minister Franz Josef Jung said the rapid reaction force would be sent to Mazar-e-Sharif in the next few months, but could be deployed to other areas in an emergency.
Last week, the US government sent letters to European states pressing them to send troops to southern Afghanistan.
"I think it would be a very big mistake if we would transfer our responsibility from the north to the south," Mr Jung said in response on Wednesday.
But speaking to a US Senate committee, Mr Gates said he would continue to be a "nag" on the issue at the informal meeting of Nato defence ministers in the Lithuanian capital, Vilnius, on Thursday.
"I worry a great deal about the alliance evolving into a two-tiered alliance, in which you have some allies willing to fight and die to protect peoples' security, and others who are not," he said.
"And I think that it puts a cloud over the future of the alliance if this is to endure, or perhaps even get worse."
Rift
Mr Gates reluctantly agreed recently to send an extra 3,200 US marines to the country, having previously suggested the extra troops should be provided by other countries.
All 26 Nato members have contributed troops to the International Security Assistance Force (Isaf), but many US allies, including Germany, France, Spain, Turkey and Italy, have refused to send significant numbers of combat forces to the south.
The decision has created a rift between them and the US, UK, Canada, the Netherlands, Denmark and Romania, whose troops have borne the brunt of the counter-insurgency fight in recent years.
Canada has been the latest to warn it could pull its contingent of 2,500 troops out of the country unless allies provide reinforcements in Kandahar province.
Mr Gates's comments were echoed by US Secretary of State Condoleezza Rice when she met UK Prime Minister Gordon Brown and Foreign Secretary David Miliband in London on Wednesday.
"The alliance is facing a real test here. And it is a test of alliance strength," she said.
Ms Rice added that people needed to understand it was not a peacekeeping mission in Afghanistan, but a fight against the resurgent Taleban.
Mr Brown earlier told MPs: "What we are looking for... is a determination on the part of all our allies to ensure the burden sharing in Afghanistan is fair."
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