Rasmussen delivered his second annual report Thursday from Brussels. He expressed concern that allies would cut back on defense spending during tough economic times.
"Our security rests on our prosperity: you can't be safe if you're broke," he said. "But in turn, our prosperity rests on our security."
Rasmussen said defense spending among NATO members has been uneven, with emerging powers spending more than established allies.
"There is a lower limit of how little we can spend on defense, while living up to our responsibilities," said Rasmussen.
U.S. lawmakers in a New Year's Day vote postponed a series of automatic budget cuts known as sequestration until the end of March. The deal delayed more than $54 billion in military spending reductions.
Outgoing U.S. Secretary of Defense Leon Panetta said "fiscal uncertainty" was one of the more immediate threats to national security.
"We have to invest to keep our societies safe," said Rasmussen. "Because security threats won't go away while we focus on fixing our economies."