South Korea and Japan are so wide apart in their positions on historical issues that it could be difficult for the two American allies to substantially improve relations even if they hold a formal summit, a U.S. congressional report said Sunday.

The Congressional Research Service issued the analysis in a report on U.S.-South Korea relations, saying the approaches of South Korean President Park Geun-hye and Japanese Prime Minister Shinzo Abe to historical issues "appear to contradict one another and are locked in a vicious circle."

"Even if the two leaders hold a formal summit, it is unclear whether the two governments will have the interest or capacity to do more than maintain ad-hoc cooperation, such as in response to aggressive North Korean actions," the report said.

The report said that Park seeks to bring Japan to a more full-throated acknowledgement and apology for its actions during the 1910-45 colonial rule and appear to link other aspects of relations with Tokyo to the history issue.

On the other hand, the report said, Abe aims to restore Japanese pride in its history by easing many signs of what many Japanese nationalists have regarded as self-flagellation, such as the portrayal of the early 20th century in history textbooks.

Frayed relations between Seoul and Tokyo limit U.S. policy options in Northeast Asia and cause tension between Washington and one or both of the two allies, and U.S. policymakers have encouraged both allies to come to terms with each other, the report said.

Relations between Seoul and Tokyo have been strained badly for years. The two countries have not held a formal bilateral summit since 2012 as Tokyo has refused to accept Seoul's demand that it formally apologize to and compensate sexual slavery victims, known as "comfort women."

Strained 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 as a key pillar for President Barack Obama's "pivot to Asia," aimed in part at keeping a rising China in check.

Meanwhile, the CRS report said that Korea-U.S. relations have been at their most robust point since the formation of the alliance in 1953, but their ties could be tested in coming months over how to handle Seoul-Tokyo relations and how to respond to Japan's efforts to expand its military role.

Seoul and Washington could also disagree over how to deal with North Korea, it said.

"The Obama administration publicly has backed Park's trustpolitik strategy and appears comfortable letting Seoul take the lead in trying to encourage more cooperative behavior from Pyongyang," the report said.

"However, in the future, some of the cooperative elements of Park's approach toward North Korea could conflict with U.S. policy due to an inherent tension that exists in the two countries' views of Pyongyang," it said.

Washington's predominant concern is the North's weapons of mass destruction programs while Seoul's top priorities are WMD issues and promoting unification, the report said.

The North Korean Sanctions Enforcement Act, which was introduced in the House of Representatives earlier this year, could "conflict Park's initiatives, such as her stated desire to internationalize" the Kaesong Industrial Complex, the report said.

On the controversy over the U.S. desire to deploy a Terminal High Altitude Area Defense missile defense battery to South Korea, the report said the issue "has become a litmus test for Seoul's straddling the gap between Beijing and Washington."

Seoul has other concerns surrounding the issue, such as the affordability of buying its own THAAD system, the effectiveness of THAAD against North Korean missiles and the time frame when a THAAD system could become available for South Korea, the report said. (Yonhap)