There's speculation that North Korea leader Kim Jong-un may be in China on his first trip outside the isolated country since taking power.
Japanese media reported on Monday that a train possibly carrying a high-level official from North Korea has arrived in Beijing, sparking speculation that the country's leader, Kim Jong-un, may have been aboard.
The Kyodo news agency, citing unidentified sources close to the matter, said the visit - if confirmed that it was a North Korean official - would likely be to improve ties between North Korea and China.
Bloomberg reported that Kim had indeed visited China, citing three unnamed sources, in what would be his first known overseas trip since taking power in 2011 and ahead of a potential summit with US President Donald Trump.
Details of his visit including its purpose and itinerary were not yet known, Bloomberg said.
Relations between China and North Korea are frosty as Beijing has backed UN sanctions to punish Pyongyang for its nuclear and missile tests.
China is North Korea's only diplomatic ally and its most important trade partner, but Kim and Chinese President Xi Jinping have never met.
Kim, however, is expected to hold historic summits with South Korean President Moon Jae-in in late April and US President Donald Trump in May. Talks between North and South Korean officials to plan the rare inter-Korean summit are expected to take place on Thursday.
Chinese state media did not report the train's arrival in Beijing, or any North Korean visit to Beijing. There was also no mention of a visit on North Korean state media either.
A Chinese foreign ministry spokeswoman earlier told a regular press briefing that she was unaware of reports that North Korean officials were spotted at a train station in the northeastern Chinese city of Dandong, which borders North Korea, at the weekend.
Japanese broadcaster NNN showed images of a green train with yellow stripes arriving in China. Kim's father, the late North Korean leader Kim Jong-il, used a similar-looking train to travel abroad.
There was nothing out of the ordinary at the Beijing railway station when an AFP reporter visited it on Monday evening.
But the manager of a store at the plaza outside the station said there had been "unusual" activity in the afternoon.
"There were a lot of police officers outside and along the road in front of the station. The station was blocked inside," the man said.
Other shopkeepers declined to comment, saying they were not allowed to give interviews.
Train delays had fuelled speculation that a special train had arrived in Beijing.
The delays were posted on one of the railway network's accounts on Twitter-like Weibo, prompting users to post comments speculating about Kim's presence, which were later censored.
The North Korean embassy did not appear to have extra security. A police car stood idle outside one of the side entrances. The compound was quiet except for birds chirping.