President Donald Trump opened the door to a future meeting with North Korea's Kim Jong Un, offering unusual praise for the globally ostracised leader at a time of surging nuclear tensions.

Although the White House played down near-term prospects for such a meeting, Trump's conciliatory comments Monday marked a departure from his more unforgiving tone toward the North in recent weeks.

It marked the latest fluctuation as Trump's administration struggles to articulate its policy for addressing the growing threat from North Korea's nuclear program.

“If it would be appropriate for me to meet with him, I would absolutely, I would be honoured to do it,” Trump told Bloomberg News.

Clearly aware of the power of his declaration, he added: “We have breaking news.” As a presidential candidate, Trump suggested he was open to meeting Kim, but hadn't repeated the line since taking office.

Fresh missile tests by the North and its progress toward developing a nuclear weapon capable of hitting the United States have made the isolated communist dictatorship one of America's top national security concerns.

Deeming President Barack Obama's “strategic patience” with North Korea a total failure, Trump and his aides say they're taking a more aggressive approach, at times warning of potential military confrontation if the North doesn't change course. The US has even raised the possibility of a pre-emptive strike if Pyongyang conducts another nuclear test.

Yet on other occasions, Trump's administration has dangled carrots. It has spoken of restarting negotiations with the North and even suggested resuming food aid to North Korea once it starts dismantling its nuclear and missile programs.

On one point, at least, Trump and his team have been consistently clear: A solution requires involvement by China, the North's biggest economic partner. Trump is hoping China can pressure the North into a peaceful denuclearization. The administration of President Barack Obama unsuccessfully sought the same objective for years.

Trump's suggestion of admiration for Kim, however, is something entirely new.

He noted that Kim assumed office in his 20s and has held power despite efforts by “a lot of people” to take it away.

“So obviously, he's a pretty smart cookie,” Trump told CBS' “Face the Nation” in an interview that aired Sunday.

Tasked with explaining Trump's flattery, White House spokesman Sean Spicer said there would be no meeting with the secretive North Korean leader until circumstances were right and numerous conditions met. He said Kim should have to alter his government's behaviour and “show signs of good faith.” “Clearly, conditions are not there right now,” Spicer said.

But echoing Trump's gentler tone, Spicer said Kim had “managed to lead a country forward” from a young age. Spicer didn't mention that under Kim, North Korea's government remains strictly authoritarian and dissent isn't tolerated. Much of the country is impoverished and malnourished.

Sen. John McCain, R-Ariz., who has urged a tough approach to North Korea, warned that Trump would be legitimising Kim by granting a meeting to the leader of a brutal regime.

“The only time he should meet with him is to get the details of how North Korea will abandon their nuclear weapons program,” McCain told reporters.

Trump's musings about a potential meeting with Kim were reminiscent of Obama's declaration during his 2008 campaign that he'd be willing to meet without pre-condition with the leaders of North Korea, Iran, Cuba and other nations long at odds with the US Republicans and Obama's Democratic opponent, Hillary Clinton, criticised Obama for that statement. As president, Obama ultimately spoke by phone with Iran's leader and travelled to Cuba amid a historic detente with the island nation.

The US maintains no diplomatic relations with North Korea and the two countries are technically at war, as the 1950-1953 Korean conflict ended without a peace treaty. The North makes no secret of its intention to develop a nuclear weapons system capable of striking the US mainland. In recent weeks, North Korea has conducted major military drills, test-fired missiles in violation of U.N. resolutions and taken preparatory steps toward a fifth nuclear test, steps that have fuelled growing US concerns and prompted Trump to send an aircraft carrier to the area.

The president's latest comments on North Korea came as his CIA director, Mike Pompeo, was in South Korea, where tens of millions of people live within artillery range of the northern border. Secretary of State Rex Tillerson last week chaired a U.N. Security Council meeting and urged countries to ramp up sanctions and suspend diplomatic ties with the North to increase pressure.-AP