World Bulletin / News Desk

Speculation that North Korean leader Kim Jong Un was in Beijing for his first ever foreign trip was rife Tuesday after Japanese media reported the arrival and departure of a special train met by an honor guard.

If confirmed, it would mark Kim's first trip abroad since coming to power in 2011 and signal an intriguing twist in a rapid diplomatic thaw that has opened the door to separate summits between Kim and the presidents of South Korea and the United States.

Japan's Kyodo news agency reported the train's departure from a Beijing station but said it was not clear if Kim was aboard, a day after its arrival sparked rampant speculation about the mystery passenger's identity.

Foreign ministry spokeswoman Hua Chunying said that any information on rumors that Kim or another high-level North Korean official had paid a visit to Beijing would be released "in due course".

China was willing to work with North Korea to "continue to play a positive and constructive role in order to realize the denuclearization of the peninsula", she said.

Some analysts had suggested China -- the North's only major ally -- had been sidelined by Pyongyang's approaches to Seoul and Washington, but a visit by Kim would put Beijing firmly back at the center of the diplomatic scrum.

Bill Bishop, the publisher of the Sinocism China Newsletter, said Xi likely wanted to meet with Kim before a summit with US President Donald Trump, which could take place in May.

"They're concerned about being left out, with the North Koreans directly cutting a deal with the Americans that don't necessarily reflect Chinese interests," Bishop told AFP.

At the Diaoyutai guest house, where Kim's late father Kim Jong Il stayed during his visits to Beijing, there was an unusually heavy police presence with officers stationed every 50-100 meters in front of the imposing compound.

An AFP photographer saw a motorcade of limousines leave the guest house under a police escort on Tuesday morning.

There was also heightened security at two possible venues for a high-level meeting --  the Great Hall of the People and Zhongnanhai, the central leadership compound next to Beijing's Forbidden City.

South Korean media speculated that the visitor might also have been Kim's sister Kim Yo Jong, or the country's ceremonial head of state Kim Yong Nam.