After keeping it under wraps and the world guessing, China on Wednesday announced North Korean leader Kim Jong-un was indeed in Beijing, and met Chinese President Xi Jinping.

An arrival of a special North Korean train and heightened security in Beijing had led the media to believe Kim was in town but neither Beijing nor Pyongyang let on until Wednesday morning when China's state-controlled Xinhua news agency flashed the news.

The news outlet described Kim's visit from Sunday to Wednesday as "unofficial" and said the North Korean leader and Xi discussed Korean Peninsula nuclear crisis.

Kim told Xi that the situation on the Korean Peninsula was improving, as Pyongyang has taken the initiative to ease tensions and decided to talk to Seoul and Washington.

A reclusive Kim is due to meet South Korean President Moon Jae-in next month and US President Donald Trump in May on North Korea's nuclear tests.

China is North Korea's sole ally. Their friendship dates back to the Korean War when Beijing stood by Pyongyang.

However, North's testing of ballistic missiles by defying sanctions has frustrated China which is wary of a war in the neighbourhood.

"It is our consistent stand to be committed to denuclearisation on the peninsula, in accordance with the will of late President Kim Il Sung and late General Secretary Kim Jong Il," Kim told Xi.

Kim said North Korea is determined to transform the inter-Korean ties into a relationship of reconciliation and cooperation and hold summit between the heads of the two sides.

Pypongyang is willing to have dialogue with the US and hold a summit of the two countries, he said.

"The issue of denuclearisation of the Korean Peninsula can be resolved, if South Korea and the US respond to our efforts with goodwill, create an atmosphere of peace and stability while taking progressive and synchronous measures for the realisation of peace," said Kim.

Xi said positive changes had taken place on the Korean Peninsula since this year, and China appreciates the important efforts made by North Korea.

Xi said China sticks to the goal of denuclearisation of the peninsula, safeguarding peace and stability on the peninsula and solving problems through dialogue and consultation.

China calls on all parties to support the improvement of inter-Korean ties, and take concrete efforts to facilitate peace talks, said Xi, noting China will continue to play constructive role on the issue and work with all parties, including North Korea, toward the thaw of the situation in the peninsula.