Trump added to the uproar over his summit with Russian counterpart Vladimir Putin when he said on Tuesday that the people of the tiny Balkan nation of Montenegro were "aggressive" and capable of triggering World War III.

In that case, would it be reasonable to go to their defence, a Fox News journalist asked the president.

"I understand what you're saying. I've asked the same question," Trump answered.

The president's comments were in line with his overall "America First" strategy and with longstanding US doubts about the wisdom of coming to the rescue of smaller, troublesome allies in Europe at the risk of triggering a wider conflict.

A NATO official speaking to AFP made no direct reference to the controversy over Montenegro but noted that at a NATO summit in Brussels last week, Trump had re-stated Washington's commitment to the alliance.

"NATO's collective defence clause, Article 5, is unconditional and iron-clad. It means that an attack on one is an attack on all," said the official who asked not to be named.

"President Trump has made clear that the US is fully committed to NATO and our alliance is strong," he said.

"Article 5 has only been invoked once, in support of the US after the 9/11 attacks.

"This led to NATO's largest ever operation, in Afghanistan, where hundreds of thousands of soldiers from Europe, including from Montenegro, as well as Canadians, have stood shoulder to shoulder with US troops and more than a thousand have paid the ultimate price," the official added.

Montenegro joined NATO last year, cementing its turn towards the West away from Russia's vision for what Moscow sees as its traditional sphere of influence in the Balkans.