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(CNN) —  

A rising diplomatic tide really does lift all boats.

After a breakthrough on the Korean Peninsula last week, Japanese Prime Minister Shinzo Abe and Chinese President Xi Jinping discussed the situation on the phone Friday, the first time leaders of the two countries had ever done so.

“It is the first time that the Prime Minister of Japan and the President of China have held telephone talks,” Abe told reporters afterward. “This means Japan-China relations are really improving.”

He said he agreed with Xi on the importance of the inclusion of “complete denuclearization” in the Panmunjom Declaration signed last week by the leaders of North and South Korea.

In a separate call with Xi, South Korean President Moon Jae-in thanked the Chinese leader for his “support and encouragement” of the inter-Korean talks, according to Moon’s office.

The Panmunjom Declaration committed Seoul and Pyongyang to seeking a formal end to the Korean War, which never officially ended despite an armistice signed in 1953. China was a participant in the war and a signatory to the armistice agreement that ended the fighting, and its support would be necessary for a formal peace treaty to be agreed upon.

Xi and Moon said Friday they would “communicate closely and actively cooperate during the process of declaring the end of the war.”

The leaders of South Korea, China and Japan will meet Wednesday for the first trilateral talks between the three countries since 2015.

Japanese Prime Minister Shinzo Abe meets with Chinese President Xi Jinping at a 2014 forum.
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Japanese Prime Minister Shinzo Abe meets with Chinese President Xi Jinping at a 2014 forum.

While Beijing’s ties with Seoul have improved considerably as Moon has sought a diplomatic solution to the North Korea crisis, relations with Tokyo have always been more strained.

Improving relations

Bitter territorial disputes over islands in the East China Sea and long-running grievances dating back to World War II have dogged the Japan-China relationship for decades.

But amid a joint fear of being left out of diplomacy on the Korean Peninsula and regional tremors caused by President Donald Trump’s economic isolationism, China and Japan have found they have more in common than ever before.

Later this month, Chinese Premier Li Keqiang is due to visit Japan for the first time in eight years, and he is expected to meet with Emperor Akihito.

It will be the first time one of China’s top leaders has met with the Japanese emperor since Xi, then vice president, saw Akihito in Tokyo in 2009.

“There has been a significant warming of relations between the two sides,” said Rana Mitter, a professor of history at the University of Oxford. However, he pointed out the “overall strategic points haven’t really changed.”

“China is still spending a lot on the (army), navy; Japan continues to have an abiding concern that the primary purpose of the Chinese expansion is to change the rules of the regional order,” he said.

CNN’s Junko Ogura reported from Tokyo, while Ben Westcott and James Griffiths reported and wrote from Hong Kong. CNN’s Sophie Jeong contributed to the report from Seoul, South Korea.