Denuclearization of North Korea will be the top agenda item at the upcoming summit between President Moon Jae-in and U.S. President Donald Trump, a U.S. State Department official said.

Joy Yamamoto, director of the department's office of Korean affairs, made the remarks at a forum at the Washington-based think tank Center for Strategic and International Studies on Monday.

Yamamoto said that there is “no question” that negotiating denuclearization of North Korea will be the "number one topic” during the Moon-Trump summit.

She also praised the dynamism of the South Korea-U.S. alliance, but said the two countries should make efforts to improve their relationship wherever possible.

Yamamoto said that South Korea is among the U.S. allies that Trump expects to shoulder a greater portion of defense costs, and that negotiations to renew a defense cost sharing agreement signed earlier this year would commence after a review of global commitments.

Seoul and Washington signed a one-year deal in March, which increased South Korea's defense cost burden by eight-point-two percent to one-point-04 trillion won to support the U.S. military presence on the peninsula.