So where is North Korean leader Kim Jong-un now?
We're hearing that he's about halfway through a grueling 60-hour-long train ride to Hanoi,.... currently crossing eastern China.
And our Oh Jung-hee has more on who's travelling with Kim... and why he chose to take the train instead of flying there.
North Korean leader Kim Jong-un exchanged greetings with senior regime officials before boarding his heavily-fortified train.
Pyeongyang's state media revealed footage of Kim leaving for Vietnam on Sunday afternoon.
Heading to Hanoi with Kim Jong-un are eight of the regime's key officials,... including Kim Yong-chol and Choe Son-hui, who've been deeply involved in the nuclear negotiations with the U.S.
Kim Jong-un's sister and close aide, Kim Yo-jong,... and the North's Foreign Minister Ri Yong-ho are also taking the trip to Vietnam.
Noteworthy additions to the delegation are Kim Phyong-hae and O Su-yong, who're respectively in charge of personnel management and industrial affairs.
O is known as the North's core 'economy' figure, being the regime's former minister of electronics and vice minister of metals and machine building.
Kim Jong-un's wife Ri Sol-ju is not part of the entourage, implying that first lady diplomacy won't happen at the summit.
Traveling from Pyeongyang to Hanoi by train is a tough journey for Kim Jong-un as it takes about 60 hours.
Hanoi is thousands of kilometers away from Pyeongyang, but it is technically close enough for Kim's personal jet Chammae-1 to fly to.
But it seems Kim decided to take the train for safety reasons.
"While the regime has used the Chammae-1 several times, its safety is not guaranteed because it's old and small. North Korea can't borrow a Chinese plane again because of their national pride."
Some watchers point out that Kim Jong-un is trying to follow in his grandfather's footsteps.
The regime's founder Kim Il-sung took a train when he visited Vietnam in 1958.
And besides these reasons, watchers say Kim Jong-un can draw global attention to his journey to Hanoi... and add dramatic effect to his diplomacy with the United States.
Oh Jung-hee, Arirang News.
