As NATO secretary general, Rasmussen has used hawkish rhetoric calling for Moscow to "de-escalate" the crisis in Ukraine and pull back thousands of troops massed on the border.

Putin made the claim at an annual televised question-and-answer session that focused on the standoff between the West and Russia over Ukraine, where pro-Russian forces have seized buildings, raising fears of a major conflict.

Rasmussen made the recording after asking to meet Putin, the Russian leader said, adding that it happened when Rasmussen was serving as Danish prime minister, from 2001 to 2009.

"I agreed, we met and talked, and it turned out he had brought a voice recorder with him and secretly recorded our conversation and then published it in the press," Putin said.

"I couldn't believe my eyes and ears. That's some kind of nonsense, you understand."

It was not immediately clear what comments Rasmussen made public.

"He explained that he recorded this conversation for history," Putin said, adding drily: "I'm flattered, of course, but he should at least have warned me or at least asked permission to publish these talks."

"It's very important, trust, it's the key question -- trust both at a level between personalities and between states," Putin said.

In Brussels, NATO rejected Putin's claim as "complete nonsense", saying Rasmussen had made it a priority to develop ties with Russia.

Rasmussen is "very concerned that Russia continues to undermine the very principles of that partnership and all its international obligations," it said, adding that Moscow "has levelled a series of accusations against NATO which distort the facts."