North Korea's Supreme Leader Kim Jong Un is on a visit to the hermit kingdom's strongest ally China, Bloomberg reported late Monday, citing sources.

The commander-in-chief is on his first trip abroad since taking power in 2011.

Additional information is not yet available due to the sensitivity of the information, Bloomberg said.

A high-ranking North Korean official appeared to have arrived by train in Beijing, Japanese media reported earlier on the same day.

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 its neighbor at the United Nations Security Council.

Reuters was unable to immediately confirm that a top North Korean official had arrived by train.

Footage from Nippon News Network, owned by Nippon TV, showed what an announcer described as a green train carriage with yellow horizontal lines, part of a 21-car train, similar to the kind that Kim's late father, Kim Jong Il, rode when he visited Beijing in 2011.

Beijing has traditionally been the closest ally to the secretive and isolated North Korea, but Kim is due to hold summit meetings separately with its rivals South Korea and the United States.

Asked earlier in the day 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.

No one could be reached by phone at the North Korean embassy in Beijing on Monday evening.

"The government is closely communicating with relevant countries and monitoring the situation," 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.

Two sources in northeastern China also told Reuters that a North Korean visitor had crossed into Dandong by train.

The rail journey between Dandong and Beijing covers more than 1,100 kilometers. It takes at least 14 hours by ordinary service, according to Chinese railway timetables.

On Chinese social media, some residents of Dandong said that there had been high security around the train station there and said there were rumors that Kim was passing through.

Police tightened security along Beijing's 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 in progress at the Great Hall of the People, where top Chinese leaders often meet visiting heads of state.

There was a large security presence outside the Great Hall on Monday evening.

Late Monday evening, Reuters reporters saw a lengthy motorcade, including a limousine with dark tinted windows, heading down Changan Avenue in the direction of the Diaoyutai State Guest House and away from the Great Hall of the People, flanked by a police escort on motor-bikes.

Also 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. Other diplomatic sources said they were aware of the speculation that Kim was visiting but were not able to immediately confirm it.

Visits to China by 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.