SEOUL, South Korea >> President Donald Trump delivered a sharp warning to North Korea Wednesday, telling the rogue nation: “Do not underestimate us. And do not try us.”

In a speech delivered hours after he aborted a visit to the heavily fortified Korean demilitarized zone due to bad weather, Trump said he had a message for North Korean leader Kim Jong Un.

“The weapons you are acquiring are not making you safer, they are putting your regime in grave danger,” Trump told an audience of South Korean lawmakers, calling on all nations to join forces “to isolate the brutal regime of North Korea.”

“The world cannot tolerate the menace of a rogue regime that threatens with nuclear devastation,” he said.

Trump had hoped to underscore his message with an early morning visit to the DMZ, but his plans were thwarted by heavy fog that prevented his helicopter from landing at the heavily fortified border that has separated the North and South for the last 64 years.

The Marine One helicopter left Seoul at daybreak and flew most of the way to the DMZ, but was forced to turn back just five minutes out due to poor weather conditions. Reporters traveling in a separate helicopter as part of the president’s envoy saw fog through the windows, and weather reports from near the heavily fortified border showed misting conditions and visibility below one mile. Pilots, officials said, could not see the other helicopters in the air.

White House press secretary Sarah Huckabee Sanders said the president was disappointed he couldn’t make the trip. “I think he’s pretty frustrated,” she told reporters. “It was obviously something he wanted to do.”

Before he left for Asia, a White House official had ruled out a DMZ visit for Trump, claiming the president didn’t have time on his schedule and that DMZ visits have become a little cliché.

But Sanders said the visit had been planned well before Trump’s departure for Asia. The trip was kept secret for security reasons, she said.

The aborted visit came hours before Trump addressed the South Korean National Assembly as he closed out his two-day visit to the nation and headed to his next stop, Beijing.

In the speech, Trump painted a bleak portrait of life in North Korea, describing citizens as bribing government officials to leave the country just so they can work as slaves. He contrasted the poverty and desperation to thriving South Korea — home to a long list of top-rated golfers, he noted.

Trump said the U.S. will not allow its cities to be threatened with destruction, and said that, while America “does not seek conflict or confrontation,” it will not run from it, either.

“The regime has interpreted America’s past restraint as weakness. This would be a fatal miscalculation?,” Trump said. “This is a very different administration than the United States has had in the past.”

He also called on all nations to downgrade diplomatic and economic ties with the North and fully implement a series of U.N. Security Council measures, specifically calling out Russia and China, whose leaders he will meet in coming days.

On Tuesday, Trump’s first day on the Korean Peninsula, he had signaled a willingness to negotiate as he urged North Korea to “come to the table” and “make a deal” over its nuclear weapons program.

He also said he’d seen “a lot of progress” in dealing with Pyongyang, though he stopped short of saying whether he wanted direct talks.

“It makes sense for North Korea to come to the table and make a deal that is good for the people of North Korea and for the world,” Trump said at a news conference with South Korea’s president, Moon Jae-in.

He also sounded an optimistic note on disagreements with the North, saying confidently, if vaguely: “Ultimately, it’ll all work out.”

It was a striking shift in tone for a president who for months had issued increasingly dire threats to answer any hostile North Korean action with “fire and fury.” In a recent speech at the United Nations, Trump said he would “totally destroy” the nation, if necessary, and has derided Kim as “little Rocket Man.”

North Korea has fired more than a dozen missiles this year but none in nearly two months. Analysts caution against reading too much into the pause.

Ever the showman, Trump had teased that he had a surprise in store, saying at a Tuesday evening banquet that he had an “exciting day” planned Wednesday — “for many reasons that people will find out.” He did not elaborate on what turned out to be the aborted trip to the DMZ.

Visiting the border that has become something of a ritual for U.S. presidents trying to demonstrate their resolve against North Korea’s ever-escalating aggression. Every American president since Ronald Reagan, save for George H.W. Bush, has made the trip, peering across the barren north through binoculars, hearing broadcast propaganda and reaffirming their commitment to standing with the South.

Visiting the wooded, craggy terrain inside the DMZ is like going back in time to 1953. In July of that year, the Korean War armistice agreement was signed at Panmunjom, the so-called “truce village” bisected by a marker that is the official dividing line between the North and South.

There’s no public sign of any diplomatic progress between Washington and Pyongyang. U.S. officials say the back channel between the State Department and the North Korean mission at the United Nations in New York remains intact, but contacts have not been substantive other than achieving the release of American college student Otto Warmbier in June. He died days after his repatriation to the U.S.

Moon, who has been eager to solidify a friendship with Trump, said he hoped the president’s visit would be a turning point in the standoff with North Korea.