The United States sees a "mood of optimism" in relations between Japan and South Korea and expects trilateral cooperation with its key Asian allies to expand into new areas such as humanitarian assistance, outer space and response to natural disasters, a senior diplomat said recently.

In a phone interview with Kyodo News on March 30, Mark Lambert, deputy assistant secretary of state for East Asian and Pacific affairs, said the three countries also increasingly share traditional security challenges that threaten the peace and stability of the Asia-Pacific region.

"My country is more secure when (South) Korea and Japan work closely together. I think there is a mood of general optimism that I detected in both Seoul and Tokyo about new things that we could do in a number of facets together," Lambert said.