"All responsible nations must join forces to isolate the brutal regime of North Korea — to deny it any form of support, supply or acceptance," Trump said. "The longer we wait, the greater the danger grows, and the fewer the options become.”
"The weapons you are acquiring are not making you safer. They are putting your regime in grave danger," Trump said. "North Korea is not the paradise your grandfather envisioned. It is a hell that no person deserves."
Trump, who in recent days expressed openness to a deal with North Korea, said he remains open to a negotiation but only if "complete and verifiable denuclearization" is on the table.
“We will not be intimidated," Trump said. "And we will not let the worst atrocities in history be repeated here on the ground we fought and died so hard to secure."
He added, "If you want peace, you must stand strong at all times."
Trump painted a picture of horrific living conditions under Kim — a "twisted regime" he called a "cult" — describing torture and abuse and harrowing attempts to escape.
He accused China and Russia, two financial benefactors of North Korea, for being complicit in the regime's abuses.
"To those nations that choose to ignore this threat or, worse still, to enable it," Trump said, "the weight of this crisis is on your conscience." He urged all countries to sever formal diplomatic relations with North Korea and end trade and technological ties.
Administration officials have been optimistic that Trump's approach — starkly different from his predecessors’, at least rhetorically — may force the Kim regime to improve its behavior and avoid the disastrous last resort of military conflict.
On Monday in South Korea, Trump suggested he could "make a deal" with the regime, urging North Korea to "come to the table," but would not say whether he still believes direct talks are a waste of time.
"That's an option that's under consideration. The president's Cabinet is looking at this as part of the overall strategy on North Korea," McMaster said.
A senior administration official told ABC News that conversations are "ongoing" and "fluid" as to when to make the announcement, adding, "It's a matter of when rather than if."
In his first trip to East Asia as president, Trump went from South Korea to China, where he is expected to confront President Xi Jinping over North Korea, seeking concrete commitments that Beijing will do more to crack down.
"In the end, we will work things out far better than anyone understands or can appreciate," Trump said in his South Korea speech.