By Matt Spetalnick and David Brunnstrom

WASHINGTON (Reuters) - U.S. President Donald Trump abruptly canceled his top diplomat's planned trip to North Korea on Friday, publicly acknowledging for the first time that his effort to get Pyongyang to denuclearize had stalled since his summit with the North's leader.

Trump partly blamed China for the lack of progress with North Korea and suggested that talks with Pyongyang, led so far by Secretary of State Mike Pompeo, could be on hold until after Washington resolved its bitter trade dispute with Beijing.

It was a dramatic shift of tone for Trump, who had previously hailed his June 12 summit with North Korean leader Kim Jong Un as a success and said the North Korean nuclear threat was over, despite no real sign Pyongyang was willing to give up its nuclear weapons.

But Trump still kept the door open to a second summit with Kim, with whom the president recently said he has "great chemistry.""I have asked Secretary of State Mike Pompeo not to go to North Korea, at this time, because I feel we are not making sufficient progress with respect to the denuclearization of the Korean Peninsula," Trump wrote on Twitter.

The North Korean mission to the United Nations declined to comment.

Negotiations have been all but deadlocked since the June summit in Singapore. Pompeo has pressed for tangible steps toward North Korea's abandonment of its nuclear arsenal while Pyongyang is demanding that Washington first make concessions of its own.

Trump's statement came just a day after Pompeo said he would again visit North Korea and would take his new special envoy, former auto industry executive Stephen Biegun, with him.

But Trump asked Pompeo not to go during a Friday meeting and they crafted the tweets together, White House officials said.

National security adviser John Bolton, considered a leading North Korea hawk, weighed in by speakerphone during a visit to Ukraine, U.S. officials said, speaking on condition of anonymity.

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Many other key officials learned of Trump's decision by seeing the crawl across a television screen, some during a meeting on North Korea negotiations, officials said.

Some U.S. intelligence and defense officials had considered Pompeo's latest trip to be premature and said the prospects for significant progress appeared dim.

Pompeo, who would have been making his second visit to Pyongyang since the summit, had not been due to meet Kim this time.

Trump himself was still open to another meeting with Kim, in hopes of advancing the process, but was not pleased with the latest signals from North Korea, a White House official said.

South Korean Foreign Minister Kang Kyung-wha spoke to Pompeo on the phone on Saturday and expressed concern over the delay of his visit. Kang asked Pompeo to keep the momentum of dialogue with North Korea to establish denuclearization and peace on the Korean peninsula, according to the Foreign Ministry's news release.

ONUS ON CHINA

Trump put some of the onus on China, North Korea's biggest trading partner and a crucial actor in enforcing sanctions to keep pressure on Pyongyang.

"Because of our much tougher Trading stance with China, I do not believe they are helping with the process of denuclearization as they once were (despite the UN Sanctions which are in place)," Trump said on Twitter.

"Secretary Pompeo looks forward to going to North Korea in the near future, most likely after our Trading relationship with China is resolved," Trump wrote. "In the meantime I would like to send my warmest regards and respect to Chairman Kim. I look forward to seeing him soon!"

China's Foreign Ministry on Saturday expressed "serious concern" about Trump's comments, which it called "irresponsible," and said it made "stern representations" to U.S. officials. It said China would continue to play a positive role in working towards decentralization on the Korean peninsula.

Trump told Reuters on Monday he believed Kim had taken specific steps toward denuclearization and that they would "most likely" meet again.

Kelly Magsamen a former U.S. deputy assistant secretary of defense for Asian affairs, said Trump was undermining his leverage with Kim as well as that of Pompeo and his new envoy.

"It's fine to not send the Secretary due to lack of progress, but don't then also talk about how you are eager to meet with KJU and how China is thwarting you," she tweeted.

Christopher Hill, a former U.S. ambassador to South Korea, tweeted: "Looks like @realDonaldTrump has begun to worry about #NorthKorea intentions. Good decision especially if otherwise Pompeo would have returned empty handed."

Kim made a broad, vague commitment in Singapore to work toward denuclearization of the Korean peninsula but has given no sign of willingness to give up his arsenal unilaterally.

Talks since then have made little headway, with the two sides far apart on denuclearization and the U.S. insistence for this to happen before North Korea sees any sanctions relief.

Pompeo left his last visit to Pyongyang in July saying progress had been made, only for North Korea within hours to denounce his "gangster-like demands."

U.S. officials have been trying without success to persuade North Korea to detail the extent of its nuclear arsenal.

Trump's decision came just days after the U.N. nuclear watchdog reported it had not found any indication that North Korea had stopped its nuclear activities.

However, some analysts suggested Trump's move may also be a negotiating ploy. In May, Trump announced he was calling off his planned summit with Kim, citing Pyongyang's "open hostility," only to reverse himself eight days later.

The cancellation follows two days of U.S.-China trade talks in Washington that ended with no sign of progress toward resolving a deepening trade war between the world's two largest economies and no plans for more talks in the near term.

(Reporting by Matt Spetalnick, David Brunnstrom, Steve Holland, David Lawder, John Walcott and Tim Ahmann in WASHINGTON, Michelle Nichols at the UNITED NATIONS, Haejin Choi in SEOUL and Tony Munroe in BEIJING; Writing by Matt Spetalnick; Editing by Sandra Maler, Paul Tait and Helen Popper)