Senator Bernie Sanders, a leading candidate in the race to face off against Trump in 2020, said he had "no problem" with Trump's decision to meet with Kim, in a moment of high diplomatic drama in the demilitarized zone between the two Koreas.

"Sitting down with our adversaries is not a bad idea," Sanders said on ABC's "This Week." "I wish (Trump) would do that in the Middle East as well, and in the Persian Gulf."

"In the case of North Korea, if we can get rid of nuclear weapons there, that would be a very good thing."

But the Vermont senator warned Trump had "weakened the State Department" since taking office, and stressed that the US and North Korea need to "move forward diplomatically, not just do photo opportunities."

Another Democratic presidential hopeful, Senator Amy Klobuchar, told CNN's "State of the Union" that "of course, as a country, we want this to work. I think any discussions are helpful."

But she cautioned: "I don't think we know if it works until there is results."

Fellow 2020 hopeful Beto O'Rourke was more scathing -- charging Trump had "added legitimacy to Kim Jong Un" without significant concessions in return.

"Despite three years of almost bizarre foreign policy from this president, this country is no safer when it comes to North Korea," the Democratic ex-congressman said on CBS's "Face The Nation."

Former US housing secretary Julian Castro, also seeking the Democratic nomination, was just as critical.

"What's happened here is that this president has raised the profile of a dictator like Kim Jong Un," while "we haven't gotten anything out of it," he said on ABC.

Castro added, "I don't think it's fitting for the United States to continue to erratically meet with a dictator when they haven't abided by the first terms" agreed on at last year's summit in Singapore.

Trump's seemingly impromptu meeting with Kim came with negotiations between Pyongyang and Washington at a deadlock.

Their first summit in Singapore took place in a blaze of publicity but produced only a vaguely worded pledge about denuclearization.

A second meeting in Vietnam in February intended to put flesh on those bones broke up without agreement, and contact between the two sides has since been minimal.