In a move that triggered fresh recognition of the power President Donald Trump possesses to ratchet up tensions-or even start a war-with a foreign adversary in order to distract from the "legal hurricane" that now engulfs his administration, Trump announced on Friday that Secretary of State Mike Pompeo will not be visiting North Korea for scheduled high-level talks.
Some of the blame rested with China, he said, "because of our much tougher Trading stance". "I look forward to seeing him soon!"
National security adviser John Bolton, considered a leading North Korea hawk, weighed in by speakerphone during a visit to Ukraine, US officials said, speaking on condition of anonymity.
Trump has used the summit he held with Kim in Singapore as a measure of his success as president. During the summit, the pair signed a vague joint statement in which the North agreed to denuclearise, but which left almost all details undefined. "But the president has created an opening and it one that we must take by seizing every possible opportunity to realize the vision for a peaceful future for the people of North Korea". Trump said Pompeo "looks forward" to going to North Korea after the United States "trading relationship with China is resolved".
The tweets come two months after Trump and North Korean leader Kim Jong Un met in Singapore. "President Obama said that North Korea was our biggest and most risky problem", he added. "No longer - sleep well tonight!"
However, only two days ago Mr Trump said China had been a "big help on North Korea".
But there have been ups and downs in the Trump-North Korea relationship since then.
Kelly Magsamen, a former U.S. deputy assistant secretary of defense for Asian affairs now at the Center for American Progress, said Biegun's appointment would "hopefully bring focus and coherence" to U.S. North Korea diplomacy.
Trump had kept up the positive tone as recently as Tuesday at a campaign rally in West Virginia.
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. "There's been no rocket launches", he added.