Japanese media reports said a special North Korean train arrived in Beijing under unusually heavy security on Monday, suggesting a senior delegation might have been aboard.
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 traveling to China.
Japanese television network NTV and public broadcaster NHK reported the arrival of the train and said the heavy security in the city suggested a senior official was aboard.
The reports sparked speculation that leader Kim Jong Un might have been aboard the train. Kim is expected to have a summit meeting with South Korean President Moon Jae-in in late April and with U.S. President Donald Trump by May.
There had been no word of Kim planning a summit with Beijing, however. China has been one of North Korea's most important allies, but relations have grown chilly because of Kim's development of nuclear weapons and long-range missiles.
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. The NTV network said the green and yellow train appears 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.