A senior Pentagon official last week reiterated Washington’s desire to further expand trilateral defense cooperation with Seoul and Tokyo beyond the military information sharing agreement concluded among them last year.

David Shear, assistant secretary of defense for Asia and Pacific security affairs, called the intelligence sharing deal a “good first step” during his speech at an event on U.S.-Japan security in Washington. He hoped there would be opportunities for further such arrangements that he said had yet to be defined.

The U.S. has sought to strengthen three-way defense cooperation with South Korea and Japan, in part to keep a rising China in check. But strained relations between Seoul and Tokyo over historical issues have hampered such efforts.

In December, the three countries signed a memorandum of understanding on sharing sensitive military information on North Korea’s nuclear and missile programs to better counter Pyongyang’s increasing security threats. The deal, which paved the way for Seoul and Tokyo to share such intelligence via the U.S., came after the two sides failed to conclude a bilateral information sharing deal in 2012 due to negative public sentiment in South Korea about enhancing military cooperation with its former colonial ruler.

Shear’s remarks came as U.S. Joint Chiefs of Staff Chairman Martin Dempsey sought cooperation from South Korea in building an integrated air and missile defense system to be joined by Japan during his visit here last week. When he met with Defense Minister Han Min-koo on Friday, the top U.S. military officer brought attention to the issue by noting he had “productive” conversations with top South Korean military officers on the progress in building the system.

Seoul officials seem to have increasing difficulties in handling the three-way defense cooperation in the face of China’s warning and the deepening partnership between Washington and Tokyo, despite Japanese Prime Minister Shinzo Abe’s historical revisionism.

South Korea still remains indecisive on the issue of allowing the U.S. to deploy a Terminal High-Altitude Area Defense battery on its soil. China has opposed the THAAD deployment, which it suspects is aimed at undermining its strategic missile capability, as well as deterring threats from North Korea.

Abe dashed hopes for his possible apology for Japan’s wartime sexual enslavement of Korean and other Asian women in his planned speech to the joint session of U.S. Congress next month by sticking to his vague stance in an interview with a U.S. newspaper last week. He described the issue as “human trafficking,” without specifying the perpetrator, in a remark calculated to avoid unequivocally acknowledging Japan’s responsibility for the atrocity.

To Seoul’s embarrassment, however, the U.S. has heaped praise on Abe ahead of his U.S. visit set for April 26 to May 2. In his announcement Thursday on inviting Abe to address a joint meeting of Congress for the first time as a Japanese leader, House Speaker John Boehner said he was “proud to host this historic event.” In his speech last week, Shear, the Pentagon official, described Abe as “a man of great vision and a man of peace” and “very forward-leaning and future-oriented.”

Their comments might be interpreted as signaling the U.S. is shifting closer to a security partnership with Japan, though officials here want to believe there is no lessening of significance Washington puts on its alliance with Seoul.

Under these circumstances, South Korea needs to depart from walking a tightrope between the U.S. and China, at least over crucial security matters. Sitting on the fence under the cover of strategic ambiguity might be inappropriate and even detrimental to deterring Pyongyang’s evolving threats and guaranteeing Seoul’s long-term security interests.

Seoul officials are advised not to weaken the country’s defense posture against immediate and fundamental security threats by sticking to an implausible balance and a rigid stance on past issues. From this standpoint, South Korea may find some room to become more flexible in enhancing cooperation with Japan.