An unexpected third face-to-face meeting between U.S. President Donald Trump and North Korean leader Kim Jong Un on Sunday underscored that Japan is entirely out of the loop on the denuclearization issue amid the fraying of its ties with the South.

Japanese diplomats ostensibly welcome the hastily arranged Trump-Kim meeting, which most foreign affairs experts did not foresee happening, expressing hope that Washington and Pyongyang will restart denuclearization talks in a full-fledged manner.

(People in Seoul walk past a screen reporting a meeting between U.S. President Donald Trump and North Korean leader Kim Jong Un in the inter-Korean truce village of Panmunjeom in the Demilitarized Zone on June 30, 2019.)

But some Japanese government officials indicated they had not obtained information in advance about whether Trump and Kim would really meet, as their talks at the Demilitarized Zone dividing the two Koreas appear to have been arranged by the South.

Strains between Tokyo and Seoul have been running high especially after a South Korean destroyer allegedly locked its fire-control radar on a Japanese patrol plane in December last year.

A string of South Korean court rulings ordering compensation for wartime forced labor have also negatively impacted ties.

On the sidelines of the two-day Group of 20 summit in Japan's western city of Osaka through Saturday, Japanese Prime Minister Shinzo Abe and South Korean President Moon Jae In skipped one-on-one talks.

"As long as Japan-South Korea relations are bad, we might remain unable to get key information about the situation of the Korean Peninsula," a Japanese Foreign Ministry source said. "I personally think Japan should try to improve ties with South Korea."

After his nearly hour-long meeting with Kim at a South Korean facility in the border village of Panmunjeom, Trump told reporters that they agreed Washington and Pyongyang will resume denuclearization talks within weeks and that he invited the North's leader to visit the White House.

In the landmark trip to the DMZ, Trump became the first sitting U.S. president to set foot in North Korea, while highlighting his good relationship with Kim.

Critics say that Trump decided to meet Kim just because he wanted to tout results during his campaign to win re-election next year, but a senior Japanese Foreign Ministry official rejected such a view.

If U.S.-North Korea negotiations move forward, the possibility may increase that Abe will hold talks with Kim, which could lay the groundwork for resolving the issue of Pyongyang's abductions of Japanese nationals in the 1970s and 1980s, the official said.

Although Abe had suggested a future summit with Kim would not be possible without a guarantee of progress, he has begun to say since early May, "I myself need to face Chairman Kim without conditions" to grapple with several bilateral issues.

Japan -- North Korea's neighbor that is seriously worried about its nuclear and missile blackmail -- has, meanwhile, pursued the full implementation of international economic sanctions to force Pyongyang to discard its weapons of mass destruction programs.

A source familiar with the Japanese Foreign Ministry's thinking voiced concern that a deterioration in Tokyo's relations with Seoul would further exclude Japan from nuclear talks, as Moon has saved face as a "mediator" between Washington and Pyongyang.

"Abe has made light of South Korea too much," the source said.

Even after Abe's remarks, North Korea's state-run media have repeatedly lambasted his proposal to hold a summit with Kim, urging Japan to atone for its past military occupation and colonial rule on the Korean Peninsula.

"North Korea is reluctant to set up an Abe-Kim meeting as it wants to first make the United States ease economic sanctions and promote economic cooperation with South Korea," the source said.

"Under such circumstances, no interaction between Japanese and South Korean leaders may deal a heavy blow to Abe's diplomacy with North Korea," he added.

Abe is the only leader who has not held dialogue with Kim among member nations of the long-stalled six-party talks on North Korea's nuclear program -- involving the two Koreas, China, Japan, Russia and the United States.

Late last year, Japan's Defense Ministry released video footage of the South Korean Navy ship's alleged locking of its fire-control radar on the Japanese Maritime Self-Defense Force plane as the two countries remain apart over what took place.

The Defense Ministry had opposed the release of the footage to avoid intensifying tensions with South Korea, but the prime minister's office pushed for it, some government sources said.

Earlier this month, Japanese Defense Minister Takeshi Iwaya effectively agreed with his South Korean counterpart Jeong Kyeong Doo that their authorities will end talks about the lock-on incident.

Japan and South Korea "should take one step further based on a future-oriented viewpoint," Iwaya told reporters after meeting with Jeong on the fringes of the Asia Security Summit, known as the Shangri-La Dialogue, in Singapore.

Nevertheless, there is little sign that Abe will work together with Moon to improve their ties.

"We should keep communication at least between the defense authorities of the two nations to tackle threats from North Korea," a Japanese Defense Ministry official said. "Unless the leaders of the two countries resume talks, the situation will get worse."

Shawn Ho, an associate research fellow at the S. Rajaratnam School of International Studies in Singapore, said, "For the foreseeable future, it does not appear that there will be a possibility of Japan coming into the picture" over Korean issues.

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