Outgoing US Defence Secretary Robert Gates says NATO is in danger of becoming irrelevant unless European countries increase their defence spending.
Mr Gates says US taxpayers cannot indefinitely assume the growing international security burden left by cuts in European defence spending.
He warns that future American leaders could decide the alliance is not worth the cost.
In his last major speech before he retires later this month, Mr Gates also said operations to protect civilians in Libya have exposed shortcomings in the military capability and political will of some NATO members.
Just 11 weeks into the operation, the BBC reports, the allies revealed they were beginning to run short of munitions. Supplies had to be brought in from the US.
Mr Gates said that since the fall of the Berlin Wall two decades ago America's share of Nato's spending had risen to 75%. The BBC's correspondent says the Defence Secretary clearly does not think that is sustainable.
"The blunt reality," Mr Gates said, "is that there will be dwindling appetite and patience in the US Congress - and in the American body politic writ large - to expend increasingly precious funds on behalf of nations that are apparently unwilling to devote the necessary resources... in their own defence."
Shortly after the speech, Norway announced that it would withdraw completely from the NATO-led operation in Libya by the start of August.