The North Korean leader was on his way to the Chinese capital Tuesday - which is believed to be his 35th birthday - for his fourth summit with Chinese President Xi Jinping as talks on a second meeting with U.S. President Donald Trump gather steam.
When North Korean leader Kim Jong Un used his New Year speech to highlight coal as a "primary front" in developing the economy, he was making a case for what analysts see as a flawed but key resource on which his country increasingly relies.
Kim was to visit through Thursday, the spokesperson said.
South Korea's Yonhap news agency, citing an unspecified source familiar with North Korea issues, earlier said a North Korean train possibly carrying a "high-level" official had crossed the border into China.
Yonhap said the train was expected to reach Beijing at about 10 am Tuesday local time.
A high-profile defection by one of North Korea's elite would be a huge embarrassment for leader Kim Jong Un as he pursues diplomacy with Seoul and Washington and seeks to portray himself as a player in worldwide geopolitics.
Progress, however, has been slow in denuclearization for months as the North calls for sanctions relief, while Washington says such concessions would not be granted until the North completely gives up its nuclear weapons.
"Kim is eager to remind the Trump administration that he does have diplomatic and economic options besides what Washington and Seoul can offer", Harry J. Kazianis, Director of Defence Studies at USA -based Centre for the National Interest said in an e-mailed statement.
Trump said there's "very good dialogue" between the US and North Korea despite the recent stalemate in denuclearization talks.More news: Alexandria Ocasio-Cortez Keeps Firing Back at Her Haters
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They discussed planning a second summit between Trump and Kim, the paper reported, fuelling speculation that Vietnam could host the event.
"We are negotiating a location", he told reporters.
Last year, Mr Kim visited the country three times, paying his respects to Mr Xi after not meeting him for more than six years following his rise to power.
"It will be announced probably in the not-too-distant future", he said, according to the BBC.
In his New Year's speech last week, the North's leader warned that he could take a new way if the USA clings to pressure and sanctions on its regime. The pair left the official residence weeks before his term was due to be up in late November, South Korean lawmaker Kim Min-ki told the AP.
Trump said: "I think at some point we are going to end up doing something great for China and great for the United States".
About 30,000 North Koreans have fled repression and poverty and settled in the capitalist South, mostly by secretly crossing over the increasingly porous border with China. North Korea is notoriously secretive about such matters.
But as worldwide sanctions have increased over the past year, coal is one of the few local resources to which Kim can turn as he tries to make good on promises to improve life in a country notorious for limited electricity, analysts and defectors say.