"Defense spending among the allies is increasingly uneven, not just between North America and Europe, but also among European allies," Rasmussen said in an annual address in Brussels.
"If these spending trends continue, we could find ourselves facing three serious gaps that would place NATO's military capacity and political credibility at risk in the years to come."
Rasmussen said the main risk was that "while some European allies will continue to acquire modern and deployable defense capabilities, others might find it increasingly difficult to do so."
This warning echoes repeated complaints from the United States that partners are not pulling their weight within the international context.
Only three NATO members -- the US, Britain and financially-battered Greece -- spend more than two percent of GDP on defense, according to the NATO annual report Rasmussen was presenting.
NATO allies are theoretically bound to maintain their defense spending commitments.
But beyond the US and Britain, only Estonia actually increased defense spending between 2007 and 2011, whereas other big European states, notably debt-challenged Italy and Spain, are now spending less than one percent of their national output.
For Rasmussen, this "would also risk weakening the political support for our alliance in the United States."
And further, "the rise of emerging powers could create a growing gap between their capacity to act and exert influence on the international stage and our ability to do so."
The former Danish premier argued that spending on defense is a vital contributor to economic growth anyway, and repeated a mantra that has sprung up during the economic downturn that smarter, multinational spending decisions shows the way forward.