Why would Kim Jong Un catch a mystery train to China?

BEIJING -- A visit to the Chinese capital by mysterious train has fueled speculation that North Korean leader Kim Jong Un carried out secret talks with China's government. The train, possibly carrying Kim or another high--level North Korean official, left Beijing early Tuesday morning.

China shares a border with North Korea and is the isolated totalitarian state's traditional ally.

CBS News correspondent Ben Tracy reports that China's Foreign Ministry insisted Tuesday morning it had "no information" to share on whether Kim was in China, and China's censors have been furiously scrubbing speculation on social media about a visit -- which, in China, tells you something.

If the North Korean leader was in Beijing, it would be the first time he's left his country since taking over in 2011.

Mystery train spotted in China 01:48

The arrival of the green train, caught by Japanese media cameras, was the first sign somebody important was coming to town. The green cars with yellow stripes looked just like the bulletproof train Kim Jong Un's father, Kim Jong Il, used when he traveled abroad. Then video appeared on social media of a large motorcade and an honor guard, apparently welcoming the mystery guest as a swarm of police kept prying eyes away.

China's official state guest house -- where visiting heads of state typically stay -- is only about five blocks from where CBS News' Tracy reported on Tuesday morning. While police were letting anybody else walk down the sidewalk, they wouldn't let journalists anywhere near it.

A visit by Kim Jong Un would be significant; he and Chinese president Xi Jinping have never met.

China has grown impatient with Kim's relentless nuclear ambitions and enforced tough international sanctions on the regime, straining the once close relationship. But now with Kim planning to meet South Korean President Moon Jae-in -- and President Trump -- at summits this spring, he may have decided it was wise to first pay a visit to an old ally.

Paul Haenle, a former White House official on China affairs and now the director of the Carnegie-Tsingua Center, says North Korea is extremely keen to get relief from the crippling international sanctions against it, and "China would be a key part of that effort."

But Haenle says Xi may also have a vested interest in a Kim visit. He may be trying to ensure he still has a seat at the diplomatic table.

"China does not want to sit on the sidelines and be a spectator in terms of what's happening here," Haenle tells CBS News.

Mending his relationship with China would be huge for Kim ahead of the summits with South Korea and the U.S. It would make North Korea less isolated, and China could use its influence to help Kim get what he wants out of the upcoming diplomacy.