U.S. officials tend to avoid making public comments on history and territorial disputes between South Korea and Japan -- two of Washington's key regional allies.

But Secretary of State John Kerry appears to be breaking the taboo.

Speaking to a group of Japanese and U.S. business leaders this week, he said Japan has "some unfinished business" regarding South Korea.

"We're all very cognizant of still some unfinished business with respect to the Republic of Korea and the need to move to the future and not be held by the past," he said Thursday night, addressing the 50th U.S.-Japan Business Conference in Washington, according to a transcript released by his department. The Republic of Korea is South Korea's official name.

He was apparently referring to longstanding stand-offs between Seoul and Tokyo over their shared history. Koreans believe Japan has yet to fully atone for its wartime atrocities, especially during the 1910-45 colonization of the peninsula.

Relations between the neighboring countries have been further strained in recent months as Japan's Abe administration is pushing to expand the nation's military activities.

The tensions between Seoul and Tokyo have taken a toll on Washington's efforts to bolster trilateral cooperation in dealing with North Korea and other regional security issues.

Kerry said Pyongyang presents a "very special challenge" to the international community, adding that cooperation with China is critical in addressing the problem.

"China above all has the ability to make the greatest difference in the choices that North Korea makes. And we have been having that dialogue very directly, and that policy is moving," he said. "And I believe it is the only way ultimately to -- the only way that we want to rationally accept -- to force the denuclearization of the peninsula, which is critical to the non-nuclearization of the entire region."

Earlier this week, the secretary also talked openly, albeit rather ambiguously, about the importance of Seoul-Tokyo reconciliation.

He said Caroline Kennedy, daughter of assassinated President John F. Kennedy, is taking up her new post as the U.S. ambassador to Japan at a critical time when there are many huge issues in play, including Washington's rebalance toward Asia and global economic competition.

Speaking at a reception at the Japanese ambassador's residence here, he also cited "the efforts to deal with North Korea, to try to reconcile even more so with the Republic of Korea, the challenges of the South China Sea, the challenges of climate change and of communications."

Victor Cha, a professor at Georgetown University who attended the reception, said he was surprised by such unusual public remarks by a top U.S. official on the matter of relations between Tokyo and Seoul. (Yonhap News)