Sharp differences on historical issues that have strained relations between Japan and South Korea require Tokyo to face up to its abusive wartime past and for Seoul to be less preoccupied with it, US experts say.
The divisions between the two main American allies in Asia have become a concern in Washington as it attempts to deepen its engagement in the region.
South Korea's ambassador to the US, Ahn Ho-young told the Heritage Foundation think tank on Tuesday that it would be difficult for relations to improve unless Japan "fairly and honestly" recognised its wrongdoings, including its treatment of so-called "comfort women" - Chinese and Korean sex slaves used by Japan's military during the second world war.
But US experts addressing the forum expressed frustration with the attitude of both South Korea and Japan. "Washington has become frustrated with both our friends. With Japan for its tin-eared, ham-fisted diplomatic approach towards resolving historic issues, and with South Korea's insistence on seeing every issue through the lens of history," said Bruce Klingner, Heritage Foundation's senior research fellow on Northeast Asia.
He said the evidence of atrocities by imperial Japan between 1910 and 1945 was "unequivocal and overwhelming" and for anyone in Japan to question Tokyo's responsibility was "morally reprehensible".
Evans Revere, a former senior State Department official, said the relationship was "often dysfunctional", undermining US efforts to forge cooperation.
South Korean perceptions that Japan lacks contrition for its militarist past have intensified since nationalist Prime Minister Shinzo Abe was elected to power in Japan a year-and-a-half ago.
Last December, Abe visited a shrine in Tokyo where convicted war criminals are among the 2.5 million honoured. Last week, two Japanese Cabinet ministers also visited the Yasukuni shrine.
South Korea is also suspicious about the Abe government's moves to reinterpret Japan's pacifist post-war constitution to allow its troops a more active role in military activities.
Dennis Blair, a former chief of the US Pacific Command, supported Japan's steps. "They should not be confused with a return to 1930s-style militarism. Japan of today is so far from that," he said.
Victor Cha, who served as director for Asian affairs under George W. Bush, said there was no prospect of "closure".