SEOUL, From a row over Japan's wartime forced labor to an unresolved military spat, a string of challenges are dimming the prospects for strategic cooperation between Seoul and Tokyo amid uncertainties over Pyongyang's denuclearization.

In recent months, strains in their ties have led to a decline in defense exchanges, exacerbated mutual distrust and cast doubts over their partnership in confronting the North's nuclear threat, a rare rallying point for South Korea and its onetime colonizer.

Tokyo's decision to skip the South Korea-based portion this week of a multilateral maritime exercise underscored the deterioration of the bilateral relationship long plagued by historical and territorial feuds stemming from Japan's 1910-45 colonization.

The decision came amid lingering tensions that flared up in December when Japan accused a South Korean warship of directing a military tracking radar at its maritime patrol aircraft in December a charge dismissed by Seoul.

Observers voiced hope that South Korean President Moon Jae-in and Japanese Prime Minister Shizo Abe could bring their frosty relations back on track should they hold one-on-one talks on the sidelines of the Group of 20 summit slated to occur in Osaka, Japan, in June.

Beyond June, it may be difficult for the two leaders to find momentum for a summit, particularly with Abe likely to focus on preparations for parliamentary elections slated for July.

"The summit, if realized, would be a symbolic event that may signal a start of the two countries' endeavors to normalize their relations," said Nam Chang-hee, a professor of international politics at Inha University.

"It may serve as a turning point that will help stop their relationship from dipping to the lowest ebb," he added.

At the center of observers' concerns is that diplomatic friction between Seoul and Tokyo is spilling over into the security realm seen as a core area of bilateral cooperation in the midst of Pyongyang's military threats and Beijing's growing assertiveness.

Amid its accusations about Seoul's naval radar operation, Tokyo's Maritime Self Defense Force (MSDF) conducted menacing low-altitude flybys close to South Korean warships in December and early this year, stoking fears of a military clash.

Tensions erupted months before the radar ruckus. Due to an intense tussle over the MSDF's use of the Rising Sun Flag symbolic of Japan's past militarism, Tokyo desisted from joining the South Korean Navy's once-in-a-decade international fleet review in October.

In a stark contrast, Japan reportedly sent MSDF Chief of Staff Hiroshi Yamamura and a key destroyer carrying the flag to the northeastern Chinese city of Qingdao last week for a fleet review marking the 70th anniversary of the founding of the Chinese People's Liberation Army Navy.

The optics of the regional competitors moving to mend fences came as Tokyo is embracing a pragmatic diplomatic approach amid uncertainties over U.S. President Donald Trump's new brand of foreign policy that seems to prize American interests over other strategic values.

"Tokyo's move to shun this week's naval exercise in Korea is raising a question of whether the current status of the Seoul-Tokyo ties is desirable for South Korea's national interests," said Park Young-june, a professor of Japanese security policy at the Korea National Defense University.

"With little tangible progress in the North's nuclear disarmament, security cooperation between South Korea and Japan should continue. ... The fact that Japan did not partake even in a peace promotion exercise bodes ill for future cooperation," he added.

The nuclear talks between Washington and Pyongyang have remained in limbo since the second summit between U.S. President Donald Trump and North Korean leader Kim Jong-un ended without a deal in Hanoi in February.

The two sides failed to bridge gaps over the scope of Pyongyang's denuclearization and Washington's sanctions relief. Since the collapse of the Hanoi summit, they have hardened their rhetoric against each other despite their show of desire to keep dialogue alive.

The undercurrents of simmering historical antagonism between Seoul and Tokyo came into the open when Japan publicly denounced South Korean court rulings last year against Japanese firms over colonial-era forced labor.

Seoul's Supreme Court has recognized the individual rights of forced labor victims to claim damages. But Japan argues that all reparation-related issues were settled in a 1965 state-to-state treaty that normalized bilateral diplomatic ties.

Tokyo appears to have been agitated by the possibility of South Korean court rulings triggering a wave of lawsuits in other countries that suffered from its past imperialism, and rehashing post-war settlement issues that it thinks are done deals.

Tokyo has requested diplomatic talks with Seoul over the issue, citing Article III of the treaty, which stipulates the two sides are to settle any dispute related to the treaty primarily through diplomatic channels. If they fail to settle it, the case can then be referred to an arbitral commission.

Seoul has yet to accept the request while maintaining that a government cannot meddle in a court decision under a democratic system that separates three powers the administration, legislature and judiciary.

Further darkening chances of a thaw in bilateral ties is friction over Seoul's decision last year to disband a Tokyo-funded foundation aimed at supporting South Korean victims of Japan's wartime sexual slavery.

Seoul has defended the decision as part of its "victim-oriented approach" and denied that the dissolution of the foundation is a scrapping of a 2015 bilateral deal to settle the humanitarian issue. Japan, however, sees the disbandment as a breach of that deal.

With the relations showing no signs of improvement, concerns have arisen that the South might have little diplomatic leverage should Japan resume negotiations with the North over normalizing bilateral ties.

Keen on settling the issue of Japanese nationals abducted by North Korean agents decades ago, Tokyo has repeatedly shown its interest in resuming dialogue with the communist state and maintaining a measure of influence over peace efforts on the peninsula.

"Should Japan and the North resume negotiations over the normalization of their ties, the South might try not to be alienated and would say it wants a stake in the negotiations because (they) are about determining the future of the Korean Peninsula," said Bong Young-shik, a researcher at the Yonsei Institute for North Korean Studies.

"But Japan might be adamant, saying it is a bilateral negotiation. ... Even if Seoul makes a conciliatory gesture, I am afraid it might be too late," he added.

Concerns have also been escalating that the South's push to play a principal role in fostering a lasting peace on the peninsula could falter amid Pyongyang's growing call for Seoul to act as the "direct party" promoting the interests of ethnic Koreans, with the United States pressuring the South to put up a united front as an ally.

Another key challenge to the Seoul-Tokyo ties comes from domestic politics and lingering nationalism in both countries, analysts said.

It remains uncertain whether Abe will come forward to enhance ties with Seoul ahead of a key parliamentary election where he needs to rally conservative voters based on his security agenda, including pushing for the installation of a regular military for a greater security role.

Source: Yonhap news Agency