Gates made his comments in Brussels, speaking to a contingent of NATO's Security and Defense Agenda assembly, the Armed Forces Press Service reported Friday.
"In the past, I've worried openly about NATO turning into a two-tiered alliance between members who specialize in 'soft' humanitarian, development, peacekeeping and talking tasks and those conducting the 'hard' combat missions -- between those willing and able to pay the price and bear the burdens of alliance commitments and those who enjoy the benefits of NATO membership, be they security guarantees or headquarters billets, but don't want to share the risks and the costs," Gates said.
"This is no longer a hypothetical worry," Gates added. "We are there today. And it is unacceptable."
Gates said just five of the 28 NATO allies, the United States, the United Kingdom, France, Greece and Albania, exceed the agreed-upon 2 percent of gross domestic product spending on defense.
"The relevant challenge for us today, therefore, is no longer the total level of defense spending by allies, but how these limited -- and dwindling -- resources are allocated, and for what priorities," Gates said.
"For example, though some smaller NATO members have modestly sized and funded militaries that do not meet the 2 percent threshold, several of these allies have managed to punch well above their weight because of the way they use the resources they have."