South Korea, Japan and the U.S. signed an agreement to solidify defense cooperation on Sunday, seeking to further bolster trilateral ties in the face of North Korean threats and Pyongyang’s increasing cooperation with Moscow.

But while the agreement seeks to institutionalize trilateral ties against leadership changes, an expert expressed concern that the lack of a formal treaty could still jeopardize cooperation in the future, particularly given historical tensions between Seoul and Tokyo.

Defense ministers Shin Won-sik, Minoru Kihara and Lloyd Austin signed the Trilateral Security Cooperation Framework (TSCF) during their first-ever joint meeting in Tokyo, with the agreement immediately going