US president-elect Donald Trump needs to clarify his views on Russia, and to understand that NATO must be treated as an alliance of shared values rather than a business, Germany's defense minister said.

"What his advisors will hopefully tell him and what he needs to learn is that NATO isn't just a business. It's not a company," Ursula von der Leyen told a ZDF talk show on Thursday. "I don't know how he values NATO."

During his election campaign, Trump threatened to abandon U.S. allies in Europe if they do not spend enough on defense.

"You can't say 'the past doesn't matter, the values we share don't matter' but instead try to get as much money out of it (NATO) as possible and whether I can get a good deal out of it," she said.

Von der Leyen advised Trump against getting too close to Russian President Vladimir Putin, noting that NATO had stood by the United States after the 9/11 attacks.

"Donald Trump has to say clearly on which side he is. Whether he's on the side of the law, peace and democracy or whether he doesn't care about all that and instead he's looking for a best buddy (with Putin)," she said.

Trump has expressed admiration for Putin, condemned by the West for his actions in Ukraine.