SEOUL, South Korea — President Trump, whose long-distance threats and insults toward North Korea have stoked fears of a nuclear confrontation, brought a message of measured reassurance to South Korea on Wednesday, calling for international resolve to isolate what he called a “rogue regime” even as he missed a chance to get an up-close look inside its borders.

Gone were the threats to rain “fire and fury” on North Korea and the derisive references to its leader, Kim Jong-un, as “Little Rocket Man” as Mr. Trump said Tuesday he saw progress in diplomatic efforts to counter the threat from the North, adding, “Ultimately, it will all work out.” On Wednesday, in a speech to the National Assembly in Seoul before leaving for China, Mr. Trump pointedly called on Beijing as well as Russia to do more to rein in the North.

“We will not allow American cities to be threatened with destruction, we will not be intimidated, and we will not let the worst atrocities in history be repeated here, on this ground we fought and died so hard to secure,” Mr. Trump said, evoking memories of the Korean War. “To those nations that choose to ignore this threat — or, worse still, to enable it — the weight of this crisis is on your conscience.”

In a speech that mostly steered clear of his usual invective toward Mr. Kim, whose grandfather Kim Il-sung was the North’s first leader, Mr. Trump nonetheless had a sharp message for him. “The weapons you are acquiring are not making you safer; they are putting your regime in grave danger,” he said. “Every step you take down this dark path increases the peril you face. North Korea is not the paradise your grandfather envisioned; it is a hell that no person deserves.”