A high-ranking North Korean official appeared to have arrived by train in Beijing, Japanese media reported, though speculation that North Korean leader Kim Jong Un is currently visiting the Chinese capital has not been confirmed.
Kyodo, citing sources close to the matter, said the visit of the official was intended to improve ties between Beijing and Pyongyang that have been frayed by North Korea's pursuit of nuclear weapons and China's backing of tough sanctions against North Korea at the United Nations Security Council.
Reuters was unable to immediately confirm that a top North Korean official had arrived by train.
Beijing has traditionally been North Korea's closest ally, but Kim is due to hold summit meetings separately with rivals South Korea and the United States.
Asked earlier at a daily news briefing about reports of an important North Korean visitor arriving at the Chinese border city of Dandong, Chinese Foreign Ministry spokeswoman Hua Chunying said she was unaware of the situation.
Nobody answered the phone at the North Korean embassy in Beijing on Monday.
"We are closely monitoring the situation and trying to gather further information through various channels, South Korea's presidential Blue House said in a statement via a messaging app.
Kyodo, citing sources, reported that on Sunday, a special train that might have carried the official passed through the Chinese border city of Dandong.
Police tightened security along Beijings main east-west thoroughfare, Changan Avenue, mid-afternoon Monday, closing off the entrances to some of the buildings which face the road.
Police also cleared out all tourists from Tiananmen Square around the same time, which normally only happens when important meetings are happening in the Great Hall of the People, where top Chinese leaders often meet visiting heads of state.
On Monday evening, the Beijing railway bureau warned on its microblog, without giving a reason, of multiple train delays of up to two hours in the Beijing region.
A source with ties to the Chinese military told Reuters that it was not possible to rule out the possibility that Kim was visiting Beijing, but cautioned this was not confirmed.
A diplomatic source told Reuters that there was heavy security around the Diaoyutai State Guest House, where some high level foreign visitors stay during visits to the city.
Visits to China by Kims late father, Kim Jong Il, were only confirmed by both China and North Korea once he had left the country.
Kim Jong Il traveled by private train during his rare visits to China or Russia under tight security. Diplomats and other sources have said Kim Jong Il avoided flying for overseas trips due to security concerns.
The younger Kim, who was educated in Switzerland, is not known to have any fear of flying and state media have shown pictures of him aboard a plane. However, he is not known to have traveled outside the country since assuming power in late 2011 after his father's death.