Kim's right-hand man, Kim Yong Chol, was due in the US capital a day after talks in New York with Secretary of State Mike Pompeo made what the US diplomat called "real progress" towards holding the planned June 12 summit.

Meanwhile, back in Pyongyang, the Korean leader re-committed his isolated state to "denuclearization," boosting hopes of what would an extraordinary diplomatic turn-around just a week after Trump threatened to cancel preparations.

Since that short-lived crisis, diplomats in both countries have conducted an intense flurry of negotiations, culminating on Thursday when Pompeo sat down in New York with Kim's envoy.

Simultaneously, Kim met Russia's Foreign Minister Sergei Lavrov and, according to official news agency KCNA, said the North's "will for denuclearization of the Korean peninsula still remains unchanged and consistent and fixed."

It is still far from clear that North Korea's vision of "denuclearization" in exchange for security guarantees and sanctions relief will prove compatible with Washington's demand for a "complete, verifiable and irreversible" end to its nuclear program.

Many expert observers expect Kim, perhaps with tacit Chinese backing, to demand that Washington also reduce its own military footprint in South Korea and loosen its guarantees to treaty ally Japan.

But Pompeo suggested things are moving in the right direction.

"It will take bold leadership from Chairman Kim Jong Un if we were able to seize this once in a lifetime opportunity to change the course for the world," he said.

"President Trump and I believe Chairman Kim is the kind of leader who can make those kind of decisions, and in the coming weeks and months, we will have the opportunity to test whether or not this is the case."

- Strategic shift -

Kim Yong Chol -- the most senior official from Pyongyang to visit the United States in 18 years -- is expected to head to Washington to present an eagerly expectant Trump with a letter from his young leader.

But Pompeo warned that this message in itself may not resolve all the issues standing in the way of the summit.

"This is a difficult, difficult challenge. Make no mistake about it. There remains a great deal of work to do," Pompeo said, citing ongoing talks in Singapore and in the demilitarized zone on the Korean border.

But he said that, after what have now been two meetings with Kim Jong Un and three with Kim Yong Chol, he believes the North is at least ready to consider addressing US demands for denuclearization.

"I believe they are contemplating a path forward. They can make a strategic shift. One that their country has not been prepared to make before. This will obviously be their decision," he said.

US officials now expect the summit to go ahead, but they want North Korea to accept that nuclear disarmament be at the heart of the discussion -- and warn that there can be no end to trade sanctions without it.

Asked whether the answer would come on Friday in the letter, Pompeo said he did not know but added "we have made real progress in the last 72 hours toward setting the conditions."

"The conditions are putting President Trump and Chairman Kim Jong Un in a place where we think there could be real progress made by the two of them meeting," he said.

Earlier, in Washington, Trump had said he was "looking forward" to reading the letter.

- Russia urges caution -

On his visit to Pyongyang, Lavrov warned against setting expectations too high, urging all sides to "avoid the temptation to demand everything and now."

The veteran Russian envoy passed on greetings from President Vladimir Putin to Kim and invited him to visit Russia, the Russian foreign ministry said.

Russia is the latest major nation to reach out to North Korea since Trump accepted Kim's proposal for a summit. Kim has already had two meetings each with Chinese leader Xi Jinping and South Korean President Moon Jae-in.