Chinese President Xi Jinping and Kim Jong Un met in China for the second time in six weeks on Tuesday, signalling their warming ties ahead of the North Korean leader's expected summit with US President Donald Trump.

Shortly after the meeting was made public, Trump tweeted that he would speak to his “friend” Xi about North Korea later on Tuesday.

Kim's trip to the northeastern port city of Dalian was his second visit to China since March, highlighting efforts by the Cold War-era allies to mend relations that have chilled as Beijing has supported UN sanctions over Pyongyang's nuclear activities.

Beijing is keen to avoid being left out in the cold in a whirlwind of diplomacy that has led to Kim's historic summit last month with South Korean President Moon Jae-in and his expected meeting with Trump in June.

But Kim's second trip abroad in such a short time — after having never left North Korea since coming to power in 2011 — shows that Beijing still has a central role to play in the diplomatic shuffle.

“After the first meeting between me and Comrade Chairman (Kim), both China-DPRK relations and the Korean peninsula situation have made positive progress. I feel happy about it,” Xi said, according to the official Xinhua news agency.

North Korea's official KCNA news agency said Xi and Kim “exchanged warm greetings, unable to hold back joy at meeting again”.

“The respected leader (Kim) also expressed pleasure at the bilateral ties that are enjoying a new heyday, and praised that the high-level exchanges and strategic communication between the two countries have reached an unprecedented level,” according to KCNA.

For his part, Trump tweeted that “the primary topics” of his discussion with Xi “will be Trade, where good things will happen, and North Korea, where relationships and trust are building.”

Chinese state broadcaster CCTV showed Xi and Kim taking a seaside stroll and holding talks in a conference room with several officials, while Xinhua said the two leaders met on Monday and Tuesday.

Xi said he was willing to meet Kim again, and backed North Korea's adherence to the denuclearisation of the peninsula and the dialogue between Pyongyang and Washington, Xinhua said.

“China is willing to continue to work with all relevant parties and play an active role in comprehensively advancing the process of peaceful resolution of the peninsula issue through dialogue, and realising long-term peace and stability in the region,” Xi said.

The Chinese leader also voiced support for North Korea “shifting its strategic focus to economic construction”.

Xinhua said Kim spoke “highly of Xi's profound vision and extraordinary wisdom” and “expressed his gratitude to China” for its contribution to the denuclearisation and peace efforts.

Excess baggage of history as South Korea's Moon heads to Japan

Chinese Premier Li Keqiang, meanwhile, will take part in a trilateral summit with Moon and Japanese Prime Minister Shinzo Abe in Tokyo on Wednesday, with the North Korean issue high on the agenda.

When Moon Jae-in heads to Japan on Wednesday he will be the first South Korean leader to do so in more than six years, but while the neighbours are both market democracies and US allies facing similar threats, analysts say their relationship is mired in the past.

Besides attending the trilateral meeting, Moon will hold a separate summit with Prime Minister Shinzo Abe.

Seoul and Tokyo face a common threat from nuclear-armed North Korea, and have both been on the receiving end of Beijing's economic muscle-flexing in recent years.

But despite their shared interests and outlooks, similar difficulties and extensive economic connections, their relations are marred by disputes over history and territory.

Koreans maintain a deep resentment over Japan's colonial rule of the peninsula from 1910 to 1945 and its abuses, including the wartime sex slaves euphemistically known as “comfort women”, and say Tokyo has not expressed sufficient remorse.

South Korean national identity is rooted in the struggle for independence from Tokyo, and the history is prominent in education, monuments and culture.

Sporting contests between the two are tense affairs, and aside from North Korea, Japan almost always ranks as South Koreans' most disliked country in opinion polls.

For its part Tokyo believes that all such issues were resolved through a treaty to normalise relations in 1965, which included massive economic aid to develop the South, at the time still recovering from the ravages of the Korean War.

Moon himself told Japan's Yomiuri Shimbun newspaper in an interview published Tuesday that he supported “future-oriented cooperation”, separate from the issues of history.

But at the same time, he said that “true reconciliation” was not possible unless a “sincere self-reflection and an apology from the bottom of the heart must be conveyed to and received by the victims”.

Analysts say the two countries should try to draw a line under the past in favour of “more diplomatic options”.