Donald Trump has declared NATO is "no longer obsolete", saying he is committed to the military alliance.
Before taking office in January, Mr Trump questioned its relevance, saying the organisation, established after World War Two, was "obsolete because it was designed many, many years ago".
And "the (member) countries aren't paying what they're supposed to pay", he said.
But the President has made a U-turn, describing the alliance as a "bulwark of international peace and security".
He has again called on alliance members to spend 2% of their GDP on defence within a decade, saying they "must pay what they owe".
If they "pay their fair share rather than relying on the US to make up the difference, we will all be much more secure", Mr Trump told reporters after White House talks with NATO chief Jens Stoltenberg.
At a news conference, the President also admitted the US was "not getting along with Russia at all" and relations between the two global powers may be at an "all-time low".
He said he hoped he can improve ties with Russian President Vladimir Putin but "we're going to see what happens".
Relations have become strained over Moscow's support for President Bashar al Assad, whose Syrian forces allegedly carried out a chemical attack on civilians.
Russia has vetoed a UN Security Council resolution demanding a swift investigation into the attack.
Mr Trump said: "The secretary-general and I had a productive discussion about what more NATO can do in the fight against terrorism.
"I complained about that a long time ago and they made a change and now they do fight terrorism.
"I said it was obsolete, it's no longer obsolete."
So far, only five of the 28 NATO members, including the UK, spend 2% of their GDP on defence, but the number is expected to rise next year.
Mr Stoltenberg said NATO provides crucial support to the coalition fighting Islamic State, as well as help training soldiers and intelligence sharing.
He says NATO has committed to do more in the global fight against terrorism and is committed to ensuring defence costs are split more fairly.