U.S. President Donald Trump’s attempt to make an unannounced visit to the heavily fortified border separating North and South Korea was aborted on Wednesday after dense fog prevented his helicopter from landing, officials said.

Trump tried to travel to the Demilitarized Zone (DMZ) — the doorstep of the North Korean nuclear standoff — as he prepared to wrap up a 24-hour visit to ally Seoul with a major speech to lawmakers on the North Korean threat.

He was then due to fly to China for the next leg of his 12-day tour of Asia.

However, Trump and his entourage had to turn back when the weather made it impossible for his helicopter to land in the border area, the White House said.

Trump was disappointed he was unable to visit the DMZ, which was planned secretly to show the strength of the U.S.-South Korean alliance, said White House spokeswoman Sarah Sanders.

South Korean President Moon Jae-in was also forced to abandon his effort to accompany Trump to the site, where visits by American leaders are often seen by North Korea as provocations.

“The fact that they were still planning for it showed the strength of the alliance,” Sanders said.

She later said of Trump: “He’s actually pretty frustrated.”

A visit to the DMZ, despite his aides’ earlier insistence he had no plans to go there, would have had the potential to further inflame tensions with North Korea.

Trump had dialled back some of his bellicose rhetoric towards North Korea on Tuesday and instead took more of a carrot-and-stick approach, warning Pyongyang of the U.S. military buildup he has ordered in the area but also offering it a diplomatic opening to “make a deal”.

While Trump presented no specific solution to his toughest global security challenge, he spoke in a more conciliatory tone on Tuesday at a time of growing fears across east Asia of the prospects for military conflict.

It contrasted markedly with his earlier threats to “totally destroy” North Korea if it threatened the United States, and the personal insults he exchanged with North Korean leader Kim Jong Un after the North’s most recent missile and nuclear tests.

It remains to be seen, however, whether the unpredictable Trump will build on this approach or return to the more confrontational language that has characterised his handling of the North Korean issue.

Trump’s official “talking points” for his Asia tour showed that he intends to use Wednesday’s speech to South Korean lawmakers in part to contrast South Korea’s “amazing rise” with North Korea’s “sad, backward state” and to urge resolve in the South against Pyongyang, according to a confidential document reviewed by Reuters.

He was also expected to condemn Pyongyang for its poor human rights record.

Trump will then fly to Beijing where, according to senior administration officials, he will try to convince a reluctant President Xi Jinping to squeeze North Korea further with steps such as limits on oil exports, coal imports and financial transactions.

Previewing his Beijing visit, Trump told Tuesday’s news conference in Seoul that China and North Korea’s other giant neighbour, Russia, were among countries whose cooperation will be crucial in getting North Korea to rein in its nuclear and missile programmes.

“President Xi … has been very helpful. We’ll find out how helpful soon,” Trump said.

However, it is far from clear if Xi, who has just consolidated his power at a Communist Party congress, will agree to do more.

China says its leverage over Pyongyang is exaggerated by the West, and points to its support in the U.N. Security Council for recent sanctions on North Korea as evidence that it is doing all it can to curtail the isolated North’s nuclear and missile tests.

“On this issue, China’s position and stance is already very clear and staunch,” Chinese Foreign Ministry spokeswoman Hua Chunying said on Monday. “Everyone can see clearly that we don’t need anybody to tell us what we should be doing.”

With Trump appearing to crack open the door to diplomacy with North Korea — something that China has long urged — he may have a better chance of securing further promises to intensify economic pressure on North Korea, which relies on Beijing for more than 90 percent of its trade.

North Korea’s Kim, however, has seemed willing to risk snubbing China when he deems it useful as he pursues development of a nuclear-tipped missile capable of hitting the U.S. mainland.

At the same time, Xi may be mindful that Trump has held off on trade actions against China that he loudly threatened during the 2016 presidential campaign to give Beijing more time to make progress on North Korea.

For his part, Xi will also be looking to maintain the good personal chemistry the two leaders developed when Trump hosted him at his Mar-a-Lago estate in Florida in April.

The “bromance” is set to continue when Xi returns the favour by laying on a lavish welcome for Trump’s visit.

Trump is expected to go to the Forbidden City, possibly guided by Xi, and participate in an inspection of Chinese troops, although China has released few other details.