The United States should intervene more forcefully in improving relations between South Korea and Japan, an American expert said, stressing that tensions between the two key allies are hindering the U.S.'s ability to deal with North Korea, China and other challenges.

Mark Manyin, an Asian affairs specialist at the U.S. Congressional Research Service, made the point in a paper to the Council on Foreign Relations, arguing that forceful intervention carries risks, but the "costs of nonintervention are rising."

"Continued animosity between the United States' two Northeast Asian allies remains a problem for Washington, hampering its ability to deal with the challenges posed by North Korea, China, and a host of nontraditional security threats," Manyin said.

The Seoul-Tokyo tensions jeopardize the Barack Obama administration's rebalance to Asia and constrain Washington's influence in East Asia by limiting joint contingency planning, trilateral coordination of crisis management, and the ability to address the challenge of China's rise, he said.

The expert said that the U.S. has occasionally played the role of a referee and sometimes a commissioner over the past decade, but neither role has been undertaken to help expand Japan-South Korea cooperation.

"In order to implement a new approach, leaders in all three countries should make the Japan-South Korea relationship a higher priority," he said. "However, forceful U.S. involvement is almost certain to be necessary to produce results, given the mutual mistrust between Tokyo and Seoul."

Although wading directly into the morass of Japan-South Korea tensions carries risks, Manyin said that the changes in Northeast Asian international relations are driving up the costs of letting the relationship between the two most important U.S. allies in Asia stagnate or deteriorate further.

"A failure to create the conditions for more robust Japan-South Korea cooperation and deeper U.S.-Japan-South Korea trilateralism will hinder U.S. ability to deal with North Korea, respond to the challenges posed by China, and plan for a future Northeast Asia that is more democratic, peaceful, and stable," he said.

Relations between Seoul and Tokyo have been badly strained for years, due mainly to Tokyo's attempts to whitewash its wartime atrocities and colonial occupation, especially its sexual enslavement of women for its troops during World War II.

Last month's first-ever summit between South Korean President Park Geun-hye and Japanese Prime Minister Shinzo Abe raised hopes for the improvement of their ties as the two leaders agreed to accelerate efforts to resolve the sexual slavery issue.

But follow-up negotiations have made little headway.

Frayed relations between the two allies have been a key cause for concern for the U.S. as it seeks to bolster three-way security cooperation seen as a key pillar for President Barack Obama's "pivot to Asia" initiative aimed, in part, at keeping a rising China in check. (Yonhap)