Kim Yong Chol, a veteran power player and a member of the young autocrat's inner circle, was due to arrive on an Air China flight from Beijing at 2:20 pm (1820 GMT), becoming the most senior North Korean official to visit the United States in 18 years.

He was due to meet US Secretary of State Mike Pompeo, who was travelling up from Washington, for talks later Wednesday and on Thursday to finalize planning for a June 12 summit designed to end a nuclear stand-off that once threatened to plunge Korea back into war.

US and North Korean envoys have also been meeting in Panmunjong in the demilitarized zone between North and South Korea, and an American "pre-advance" team is in Singapore to make logistical arrangements for the rapidly planned meeting.

But with 13 days to go, the talks between General Kim and Pompeo, the former CIA chief who pioneered the latest round of face-to-face meetings, are the highest-level effort to stabilize the on-again, off-again process of getting two unpredictable leaders to the table.

"If it takes place on June 12th, we'll certainly be prepared. If it for some reason takes place at a later date, we'll be prepared for that as well," White House spokeswoman Sarah Sanders said Tuesday.

Earlier this month, Trump suddenly but only briefly announced an cancellation of the summit after a North Korea issued a sharp rebuke of what it saw as threatening language for the US side, and Sanders warned talks could be postponed if Kim is not serious about disarmament.

"Again, denuclearization has to be on the table and the focus of the meeting. And the president has to feel like we're making progress on that front. And the only one that will make that determination will be the president," she said.

But the summit appears increasingly likely to go ahead, amid a flurry of international diplomatic activity.

Moscow said Russian Foreign Minister Sergei Lavrov would travel to North Korea on Thursday to discuss Pyongyang's nuclear program. Pompeo, meanwhile, called his South Korean and Singaporean counterparts over the weekend.

Japan is also keenly watching summit preparations. Trump will meet its Prime Minister Shinzo Abe in Washington on June 7.

On Sunday US negotiators, headed by Washington's ambassador to the Philippines Sung Kim, began meeting North Korean counterparts in the truce village of Panmunjom that divides the two Koreas.

"They plan to have additional meetings this week," Sanders said.

Kim Yong Chol will be the most senior North Korean on US soil since Vice Marshal Jo Myong Rok met then-president Bill Clinton in 2000.

The general has played a front-seat role during recent rounds of diplomacy aimed at ending the nuclear stalemate on the Korean peninsula.

He sat next to Trump's daughter Ivanka, who is also a White House aide, during February's closing ceremony for the Winter Olympics in South Korea, an event that was seen as a turning point in the nuclear crisis.

He also accompanied Kim Jong Un on both of his recent trips to China to meet President Xi Jinping, and held talks with Pompeo when he travelled to Pyongyang.

- Yawning gap -

General Kim's journey to the US caps a frenetic few days of meetings between North Korean and American officials.

An AFP photographer saw Kim Chang Son, Kim Jong Un's de facto chief of staff, in Singapore Tuesday for preparatory discussions there.

The key task is to settle the agenda. The main stumbling block is likely to be the concept of "denuclearization" -- both sides say they want it, but there is a yawning gap between their definitions.

Washington wants North Korea to quickly give up all its nuclear weapons in a verifiable way in return for sanctions and economic relief.

But analysts say North Korea will be unwilling to cede its nuclear deterrent unless it is given security guarantees that the US will not try to topple the regime.

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AIR CHINA