BEIJING: North Korea leader Kim Jong Un has visited China, Bloomberg reported on Monday 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. Japanese media reported earlier on Monday that a high-ranking Pyongyang official appeared to have arrived by train in Beijing.

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.

The visit could not immediately be confirmed.

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 f ather, Kim Jong Il, rode when he visited Beijing in 2011.

Beijing has traditionally been the closest ally of secretive and isolated North Korea. But Kim is due to hold summit meetings separately with China`s 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 telephone 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 earlier on Monday.

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 north-eastern China also said that a North Korean visitor had crossed into Dandong by train.

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