US President Donald Trump has announced that Washington's chief diplomat, Secretary of State Mike Pompeo, is on his way to North Korea to prepare for a landmark nuclear summit, as a flurry of diplomacy with Pyongyang at the centre gatheres pace.

Mr Trump and North Korean leader Kim Jong-un are expected to meet later this month or early in June for talks that would have been nearly unthinkable a year ago, when the two leaders were trading barbs and Pyongyang tested a barrage of missiles.

But after months of saber-rattling, and a historic summit between Mr Kim and South Korean President Moon Jae-in, the mood is decidedly more upbeat, and Mr Trump has said a date and location are already set - though he did not divulge any specifics.

"At this very moment, Secretary Pompeo is on his way to North Korea in preparation for my upcoming meeting with Kim Jong-un," Mr Trump said in a televised address from the White House.

Mr Pompeo, who met Mr Kim in Pyongyang about a month ago while he was still director of the CIA, will arrive in the North in "virtually an hour," Mr Trump said.

"We have our meeting schedule. We have our meeting set. The location is picked. The time and date.

"Everything is picked. And we look forward to having a very great success," he said.

"We think relationships are building with North Korea. We will see how it all works out. Maybe it won't. But it can be a great thing for North Korea, South Korea and the entire world."

In addition to planning the summit, Mr Pompeo has been pressing the regime to release three detained US citizens. Mr Trump has said to "stay tuned" on the issue.

"We will be listening for signs from North Korea that things have substantially changed since Kim declared on New Year's Eve that he would mass produce nuclear warheads and the means to deliver them," a senior State Department official traveling with Mr Pompeo said.

"We're going to be listening for - have they turned the page and are they ready to address areas that directly affect American national security? The secretary is clear-eyed about not repeating the mistakes of the past."

Mr Trump's announcement came after talks in northeast China between Mr Kim and China's Xi Jinping - their second meeting in six weeks - at which Mr Kim reiterated his commitment to the denuclearisation of the Korean peninsula.

Mr Kim's unannounced trip to the northeastern Chinese port city of Dalian was his second visit to China since March, highlighting efforts by the Cold War-era allies to mend relations that cooled as Beijing supported United Nations sanctions over Pyongyang's nuclear and ballistic missile programmes.