Prime Minister Shinzo Abe, President Moon Jae-in and Chinese leaders pledged to deepen cooperation at a trilateral summit in China's southwest city of Chengdu.
China hosted the leaders of squabbling neighbours South Korea and Japan for their first meeting in over a year on Tuesday, flexing its diplomatic muscle with America's two key military allies in Asia and seeking regional unity on how to deal with a belligerent North Korea.
The gathering, held in the southwest city of Chengdu, was held with the clock ticking on a threatened "Christmas gift" from North Korean leader Kim Jong-un that could reignite global tensions over its nuclear programme.
Kim has promised the unidentified "gift" – which analysts and American officials believe could be a provocative missile test – if the US does not make concessions in their nuclear talks by the end of the year.
The gathering also featured the first bilateral meeting between South Korea's Moon Jae-in and Japanese Prime Minister Shinzo Abe in 15 months.
Ties between their two nations have hit rock bottom in recent months over trade issues and other disputes related to decades of bitter wrangling over Japan’s 1910 to 1945 occupation of the Korean peninsula.
The US has frequently urged its two allies to bury the hatchet – worried that their poor relations were complicating diplomacy in Asia – although it has held off on direct mediation.
Japan, S Korea relations
China is appearing to fill that void with the Chengdu event.
"As the region's major power, China hopes to show its diplomatic presence to the world by bringing the Japanese and South Korean leaders to the same table," said Haruko Satoh, professor and expert on Chinese politics at Osaka University.
At separate lead-up meetings in Beijing on Monday with Chinese President Xi Jinping, Moon and Abe both spoke of China's diplomatic heft in the region.
Before leaving for China, Abe told reporters that links with Seoul remained "severe", but said the Chengdu meeting was important given the regional issues at stake.
Abe and Moon were photographed smiling and shaking hands during the meeting.
The relationship between Japan and South Korea is overshadowed by the 35 years of brutal colonisation by the Japanese – including the use of sex slaves and forced labour – that is still bitterly resented today.
Ties began a downward spiral in recent months after a series of South Korean court rulings ordering Japanese firms to compensate wartime forced labour victims.
These infuriated Tokyo, who insisted the matter had been settled by a 1965 treaty between the two countries.
Seoul then threatened to withdraw from a key military intelligence-sharing pact, although it reversed course in November and agreed to extend it "conditionally".
After the trilateral meeting, both Japan and South Korea urged the resumption of talks between Pyongyang and Washington, which have been largely deadlocked since a second summit between President Donald Trump and North Korean leader Kim in Hanoi collapsed at the start of this year.
"It is important that we give our utmost support to the US-North Korea process toward complete denuclearisation of North Korea," said Abe.
The leaders of the three countries also promised to help promote dialogue over the denuclearisation of the Korean peninsula.
Chinese Premier Li Keqiang told a press conference they had "reaffirmed that denuclearisation of the peninsula and the lasting peace in East Asia are the common goal of the three countries".
In an effort to pressure the US, the North has conducted a series of short-range missile tests and threatened to go further with its "Christmas gift".
If the North fired an intercontinental ballistic missile in defiance of UN sanctions, it would destroy Trump's argument that he had succeeded in reducing risks from North Korea.
Former US national security adviser John Bolton, who was dismissed in September, on Monday criticised Trump's strategy and warned the North posed an "imminent" threat.
"The risk to US forces and our allies is imminent and more effective policy is required before NK has the technology to threaten the American homeland," tweeted Bolton.