"I said a long time ago that NATO had problems. Number one it was obsolete, because it was designed many, many years ago," Trump told the today's editions of German tabloid Bild and the Times of London.
While he said NATO remains "very important to me," he returned to a favourite campaign-trail complaint that some NATO allies weren't paying enough.
"We're supposed to protect countries. But a lot of these countries aren't paying what they're supposed to be paying, which I think is very unfair to the United States.
The US accounts for some 70 per cent of spending by NATO nations.
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Immediately after Trump's election victory, the organisation's secretary-general Jens Stoltenberg recalled that NATO had been at the heart of transatlantic security since its founding in 1949.
Members of NATO are pledged to defend one another if attacked, with the self-defence clause invoked only once in history -- after the 9/11 terrorist attacks on the United States.
In 2014, leaders of the 22-nation alliance -- originally founded in response to the Soviet domination of Eastern Europe -- agreed to return defence spending to their commitment of 2.0 per cent of GDP in the wake of Russian intervention in Ukraine and upheavals in the Middle East and North Africa.