The arrival of a special train in Beijing and unusually heavy security at a guesthouse where prominent North Koreans have stayed in the past have raised speculation that Kim Jong Un is making his first visit to China as the North's leader.
Mr Kim has summits planned with South Korean President Moon Jae-in in late April and with US president Donald Trump by May.
While there has been no word of a similar meeting with Chinese leaders, China has been one of North Korea's most important allies even though relations have recently chilled because of Kim's development of nuclear weapons and long-range missiles.
A vehicle convoy entered the Diaoyutai State Guesthouse in Beijing this evening and a military honour guard and heavy security were seen later.
That followed reports from Japanese network NTV and public broadcaster NHK of a special North Korean train arriving in Beijing under unusually heavy security.
A spokeswoman for the Chinese Foreign Ministry said she was not aware of the situation and had no further comment.
North Korea's state-run media had no reports of a delegation travelling to China.
South Korea's presidential office said it cannot confirm reports that the train carried Kim nor a separate report that Kim's sister was onboard.
South Korean analysts were doubtful the visitor is Kim Jong Un.
Since succeeding his father as leader in 2011, Kim has touted an image of his country as diplomatic equal to China and it is unlikely he would sneak into Beijing for his first face-to-face meetings with the Chinese leadership, the experts said.
They said it's more likely Mr Kim sent a special envoy, possibly his sister Kim Yo Jong, to appease a traditional ally ahead of his planned meetings with the presidents of South Korea and the United States.
The envoy could potentially seek Chinese commitment for future support should North Korea's talks with rivals fall through, said Du Hyeogn Cha, a visiting scholar at Seoul's Asan Institute for Policy Studies.
"North Korea doesn't want to send a message that China has been pushed to the back as it makes diplomatic approaches to the United States and South Korea," said Mr Cha, saying the visit could be part of the North's effort to gain leverage in the talks with South Korea and the United States.
"The special envoy could discuss this possibility with Chinese officials, asking China not to press too hard with sanctions if that happens."
Heavy security was reported at the Friendship Bridge before the train passed from North Korea to China, and there were reports of it passing through several stations on the way from North Korea to Beijing.
NTV reported the green and yellow train appeared very similar to the one that former North Korean leader Kim Jong Il, Kim Jong Un's late father, took to Beijing in 2011 and has 21 cars.
A video that aired on NTV also showed a motorcade of black limousines waiting at the train station and rows of Chinese soldiers marching on what appeared to be a train platform. The video did not show anyone getting off the train.
White House spokesman Raj Shah said Monday the US could not confirm reports that Kim was visiting China.