By Agence France-Presse
Trump's comments, which a former US ambassador to NATO decried as a "gift to Putin," came in an interview with Fox News when he was asked about Article 5, NATO's common defense clause which states that an attack on one member is an attack on all.
"Why should my son go to Montenegro to defend it from attack?" Fox host Tucker Carlson asked.
Trump responded: "I understand what you're saying. I've asked the same question."
"Montenegro is a tiny country with very strong people... They're very aggressive people. They may get aggressive, and congratulations, you're in World War III," he added.
Montenegro, a former Yugoslav republic with a population of about 630,000, joined NATO last year, becoming is 29th member. Its military only numbers about 2,000 personnel.
The only time Article 5 was ever invoked was by America after the September 11, 2001 terror attacks conducted by al-Qaeda.
Nearly 17 years on, NATO troops are still operating in Afghanistan after the US-led invasion seeking retribution for the nation's harboring of the terror group.
"Trump sows further doubt whether the US under his leadership would defend our allies. Another gift to Putin," Nicholas Burns, who was US ambassador to NATO after the 9/11 attacks, wrote on Twitter.
Senior Republican leader Senator John McCain, who has called Monday's summit with President Vladimir Putin in Helsinki a "tragic mistake," said Trump was doing exactly what the Russian strongman wanted.
"The people of #Montenegro boldly withstood pressure from #Putin's Russia to embrace democracy," McCain said on Twitter.
"The Senate voted 97-2 supporting its accession to #NATO. By attacking Montenegro & questioning our obligations under NATO, the President is playing right into Putin's hands."
But State Department spokeswoman Heather Nauert said Trump has been unequivocal in his support of NATO's collective defense.
"The summit declaration that came out at the end of the summit stated clearly that any attack against one ally would be regarded as an attack against all," Nauert told reporters.
- NATO as 'welfare agency' -
Montenegro's relations with Russia have deteriorated in recent years as the Balkan nation moved to join NATO. It also hopes to join the European Union, much to the dismay of Putin, who sees the West perennially creeping toward Russia.
Moscow has been accused of meddling in Montenegro's elections, and a failed 2016 coup was allegedly planned by pro-Russian militants.
Richard Haass, the president of the Council on Foreign Relations, called Trump's remarks "extraordinary."
"It is not just that the president throws Montenegro under the bus; he makes the US commitment to NATO conditional and makes clear his discomfort w Article 5 and collective security, the core of the alliance," Haass wrote on Twitter.
With such a small military, it is difficult to know what Trump was referring to when he called Montenegro's people "very aggressive."
The Pentagon did not immediately comment.
Some observers derided Trump's claim that Montenegro is aggressive, re-circulating online a video of last year's NATO summit where the US president appeared to shove Montenegro's prime minister aside at a photo shoot.
Doug Bandow, a senior fellow at the libertarian Cato Institute, said Trump's comments more likely reflected his anger at NATO than any particular Russian concerns about Montenegro.
Trump has repeatedly railed against NATO, accusing its members of not doing enough to fund their militaries and an over-reliance on America.
"It raises a proper question, which is has NATO become kind of a welfare agency as opposed to a security agency'" Bandow told AFP.
"I don't quite understand the notion that the Montenegrins are so aggressive and with their two thousand men military that are likely to start World War III."
Bandow noted that Montenegro joined NATO on Trump's watch.
"This president allowed that to move forward last year. So if you allow it to happen, why are you complaining about it?" Bandow said.
John Hannah, senior counselor for the Foundation for Defense of Democracies and former national security advisor to vice president Dick Cheney, noted that Trump bridles at NATO because he considers it multilateralism at its worst.
The idea that a small country like Montenegro "could do something that might anger or upset a neighboring great power like Russia and automatically draw the United States into a major war runs counter to all Trump's instincts. That's not 'America First' in his eyes. It's the opposite," Hannah told AFP.
"It's America being played for a sucker."