U.S. Defense Secretary Mark Esper said on Thursday that the resolution of conflicts between South Korea and Japan requires leadership from both countries, adding the United States will do what it can to push it forward.

Esper made the remarks to reporters before departing Vietnam in an apparent reference to a soon-to-expire military intelligence sharing agreement between Seoul and Tokyo.

According to the Pentagon, the defense chief said that while he understands the historical roots of friction between the two allies that date back decades, there are greater concerns that involve North Korea and China. 

Regarding negotiations for defense cost-sharing for the upkeep of U.S. forces on South Korea, Esper reportedly reaffirmed that it's "not unreasonable" to ask countries who have the means to contribute more to their own defense and to the costs associated with U.S. presence.