Reversing some of his campaign rhetoric, Trump told a joint news conference that NATO "is no longer obsolete" and hailed its role in the fight "against terrorism."
But he renewed his argument that NATO's member states need to dedicate two percent of their gross domestic product to military expenditure.
"In facing our common challenges, we must also ensure that NATO members meet their financial obligations and pay what they owe," he said.
NATO Secretary General Jens Stoltenberg said he and Trump had agreed in their talk that "NATO is a bedrock of security, both for Europe and for the United States... and therefore I welcome the very strong commitment of the United States to the security of Europe."
Increased concerns among NATO's European states about Russian military activity on its eastern flank fed into a question to Trump about strained ties with Russia.
Trump replied that "it would be wonderful... if NATO and our country could get along with Russia."
He said "we may be at an all-time low in terms of our relationship with Russia," but said it would be "a fantastic thing" if the relationship could improve with Russian President Vladimir Putin.
WATCH: US accuses Russia of Syria 'cover up'
It's time to end Syria's 'brutal civil war'
Trump told allies it was time to end Syria's "brutal" civil war Wednesday, as he branded the country's leader Bashar al-Assad a "butcher" and questioned Russia's role in a suspected chemical attack.
Trump called on allies to "work together to resolve the disaster" in Syria and thanked them for condemning Assad's suspected sarin attack in Khan Sheikhun.
"Vicious slaughter of innocent civilians with chemical weapons including the barbaric killing of small and helpless children and babies must be forcefully rejected by any nation that values human life," Trump told reporters.
"That's a butcher. That's a butcher. So I felt we had to do something about it. I have absolutely no doubt we did the right thing, and it was very, very successfully done," he added.
"It is time to end this brutal civil war, defeat terrorists and allow refugees to return home."
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Russia vetoes UN draft resolution on Syria gas attack probe
Trump's comments came shortly after Russia vetoed a UN Security Council resolution that would have compelled Damascus to cooperate with an investigation of the attack.
Trump said it was "certainly possible" that Russia President Vladimir Putin knew about the attack, blamed on Assad, indicating Russian officials were present at the source airbase, which Trump later bombed.
"I would like to think that they didn't know, but certainly they could have. They were there. So we'll find out," he said.
Trump also praised China for abstaining during the UN vote. He met President Xi Jinping last week in Florida and spoke again to the Chinese leader on Tuesday.
"I think it's wonderful that they abstained," he said. "We're honored by the vote. That's the vote that should have taken place."
