China has confirmed North Korean leader Kim Jong-un has visited Beijing, and says it won a pledge from him to work towards denuclearising the Korean Peninsula.

Key points: Kim Jong-un reportedly said North Korea is "committed to denuclearisation"

Kim Jong-un reportedly said North Korea is "committed to denuclearisation" Donald Trump is briefed on the meeting

Donald Trump is briefed on the meeting Xi Jinping accepts an invite to North Korea, reaffirms friendship

Beijing announced this morning that Mr Kim had met President Xi Jinping during what the official Xinhua news agency called an unofficial visit from Sunday to Wednesday.

The trip was Mr Kim's first known journey abroad since he assumed power in 2011 and is believed by analysts to serve as preparation for upcoming summits with South Korea and the United States.

Beijing has traditionally been the closest ally of secretive North Korea, but ties have been frayed by the North's pursuit of nuclear weapons and China's backing of tough UN sanctions in response.

Xinhua said Mr Kim told Mr Xi that the situation on the Korean Peninsula was starting to improve because North Korea had taken the initiative to ease tensions and put forward proposals for peace talks.

"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 Jong-un said, according to Xinhua.

Mr Kim also reiterated his commitment to hold talks with the United States.

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

Though billed as an unofficial trip, Mr Kim's appearance in Beijing contained almost all the trappings of a state visit, complete with an honour guard and banquet at Beijing's Great Hall of the People.

Trump briefed on meeting, analysts sceptical

Reports said Kim Jong-un and his wife visited China from Sunday until Wednesday. ( Xinhua )

The two leaders also met at the Diaoyutai State Guest House, where Mr Kim's grandfather, Kim Il-sung, planted a tree in 1959 that still stands to this day.

State television showed pictures of the two men chatting amiably and Mr Kim's wife, Ri Sol-ju, getting an equally warm welcome from Mr Xi's wife, Peng Liyuan.

Beijing reportedly briefed US President Donald Trump on Mr Kim's visit and the communication included a personal message from Mr Xi to Mr Trump, the White House said in a statement.

"We see this development as further evidence that our campaign of maximum pressure is creating the appropriate atmosphere for dialogue with North Korea," it said.

The proposed summit between Mr Trump and Mr Kim poses too much of a risk, according Michael J Green, who served on the US National Security Council under president George W Bush.

"If I were in government I would have strongly opposed, and I would have probably resigned from the White House, over this summit," he told 7.30.

"Number one, it's morally distasteful for the President of the United States to meet with the leader of a truly evil, Stalinist regime that is also a criminal enterprise.

"And number two, the track record of negotiating with North Korea, and I have been involved with many of these negotiations, is pretty conclusive they will cheat and not live by any agreement."

Dr Green said China's decision to meet Mr Kim this week shows "it doesn't want to be left out of this formula, it wants to maintain influence".

"China wants to see it go forward and probably is offering Kim Jong-un a very healthy economic package," he said.

"I would not be surprised to find out that in order to incentivise Kim Jong-un, China is offering significant economic inducements.

"So already it's a setback for our campaign to put pressure on their nuclear weapons.

"Donald Trump will think he's made this brilliant breakthrough, agree to this and accidentally begin a process whereby some in South Korea, Russia and others start to dismantle our alliances in north-east Asia, and this will have an affect on ANZUS. This will affect the security of Australia.

"There is zero evidence that North Korea is going to give anything."

Analysts and former negotiators remain deeply sceptical Mr Kim is willing to give up the nuclear weapons his family has been developing for decades.

"But also, I can imagine Kim Jong-un wanting to request for sanctions relief, because we have seen that Beijing has been far more tough and has implemented sanctions far more than we have seen them do in the past," analyst Duyeon Kim told the ABC.

Mr Kim reportedly told a banquet hosted by Mr Xi the visit was intended to "maintain our great friendship and continue and develop our bilateral ties at a time of rapid developments on the Korean peninsula", according to KCNA.

But North Korea's KCNA news agency made no mention of Mr Kim's pledge to denuclearise, or his anticipated meeting with Mr Trump that is planned for some time in May.

Xi accepts invite to North Korea, confirms friendship

The train believed to have taken Kim Jong-un to China. ( NHK World )

Mr Xi accepted an invitation to visit North Korea and told Mr Kim that their traditional friendship should continue to develop.

"This is a strategic choice and the only right choice both sides have made based on history and reality, the international and regional structure and the general situation of China-DPRK ties," Mr Xi said.

"This should not and will not change because of any single event at a particular time."

Speculation about a possible visit by Mr Kim to Beijing was rife earlier this week after a train similar to the one used by Mr Kim's father was seen in the Chinese capital, along with heavy security and a large motorcade.

Improving ties between North Korea and China would be a positive sign before planned summits involving the two Koreas and the United States, a senior South Korean official said on Tuesday.

Mr Kim's father, Kim Jong-il, met then-president Jiang Zemin in China in 2000 before a summit between the two Koreas in June that year.

That visit was seen at the time as reaffirmation of close ties with Beijing.

Kim Jong-un met with South Korean officials earlier this month. ( Reuters: KCNA )

Reuters/Additional reporting by Jake Sturmer