GETTY•FOXNEWSNorth Korea news John Bolton claimed North Korea could be using Trump talks to'buy time
Japanese media is reporting a high-ranking North Korean official appears to have arrived by train in Beijing, China.
If confirmed, it would be Kim's first overseas trip since he took over power in 2011 after his father's death.
Media reports about a visit by a top-level North Korean official to China dominated the local news Tuesday, raising questions of whether it was North Korean leader Kim Jong-un.
Beijing has always been a close ally of North Korea but meetings between the two countries are not often. North Korea, too, was silent on the issue.
"We are carefully watching the situation concerning things that are now happening in Beijing, with all possibilities in mind", a Cheong Wa Dae official told reporters, declining to be named. This is similar to Kim's predecessor Kim Jong-il.
One government source suggested that it might not be the chairman who is visiting China, saying, "There is not enough circumstantial evidence to indicate that Kim Jong-un is visiting China. although the possibility can not be ruled out".
The US official, speaking on condition of anonymity, said it was unlikely Kim Jong Un would have sent his sister on such an important mission, unlike her ceremonial visit to South Korea for the Olympics.
Kim has not met any foreign heads of state since succeeding his father as North Korean leader in late 2011.
"North Korea likely wants to confirm its relationship with China and believes it has some leverage with which it can ask for things from China", said Yoo Ho-yeol, Professor of North Korean studies at Seoul's Korea University. The meeting could be a new beginning in the strained ties between Beijing and North Korea. China and North Korea had been discussing the timing of Kim's visit since the start of this year, with China insisting that North Korea must show its "willingness to work toward giving up its nuclear program", according to the official.
Reuters later reported in a dispatch from Beijing that the train left the Chinese capital in the afternoon, citing a witness.
The trains travel at a leisurely 38 miles per hour - but can whisk the leader away in a hurry in armored vehicles and even small helicopters that also are carried aboard for emergencies.
Heavy security outside Diaoyutai State Guesthouse in Beijing where top North Korean leaders have stayed on previous trips to Beijing, China. While reporters were held back behind a cordon a few hundred metres away, a motorcade reportedly carrying Kim was spotted passing by Tiananmen Square, apparently en-route to the Beijing railway station, it said.
Footage of the special train, which belongs to Kim's ruling family, has fuelled rumours the dictator has traveled to Beijing for his first foreign trip since becoming leader.
The sightings prompted a flurry of speculation that the North Korean leader could be in the Chinese capital ahead of his planned summit with South Korean President Moon Jae-in and a potential meeting with US President Donald Trump. He noted that Xi probably had advice for Kim for the upcoming summit between Kim and Trump, which will be the first-ever such talk between the two countries' leaders.
North Korea's state-run media had no immediate reports of a delegation travelling to China.