The United States "can't be overly optimistic" about the outcome of a planned meeting between President Donald Trump and North Korean leader Kim Jong Un, Adm. Harry Harris, commander of the U.S. Pacific Command, said Thursday.

With Kim reportedly committed to denuclearization, Harris said at a Senate hearing that Washington will continue to pursue a complete, verifiable and irreversible denuclearization of the Korean Peninsula.

Referring to the planned summit which Kim proposed and Trump agreed to hold by the end of May, Harris said, "As we go into this, I think we can't be overly optimistic on outcomes. We'll just have to see where it goes."

Harris said he was "encouraged" by the prospect of holding a first U.S.-North Korea summit. But he warned that Pyongyang remains "our most urgent security threat in the region."

"We've never been in a position where our president has met with a leader of North Korea, ever," he told the Senate Armed Services Committee.

"So I don't have a way to predict the future. I just think that we have to go into this eyes wide open."

The commander said Japan stands shoulder-to-shoulder with the United States on North Korean issues, including a Trump-Kim summit.

"I believe that Japan will be supportive of the outcome, and they share our concerns about the trustworthiness of North Korea," he said.

The White House has said the time and location of the summit have yet to be determined. Press secretary Sarah Huckabee Sanders said last week Trump will not meet with Kim unless he takes "concrete and verifiable actions" to meet promises he has made.

Sanders did not specify what such actions would be, but she was referring to Kim's message conveyed March 8 to Trump via a South Korean official that he is committed to denuclearization, will refrain from any further nuclear or missile tests, and recognizes that regular military exercises between the United States and South Korea will continue.

Speaking at Thursday's hearing, Harris said he believes Kim is pursuing the reunification of the Korean Peninsula under his leadership, and that he is seeking respect, status and security through the possession of nuclear weapons.

Asked how Kim would respond should the United States withdraw its 28,500 troops in South Korea, Harris said, "I believe he would do a victory dance."

"I think he'd be a happy man if we abrogated our alliance with South Korea, and with Japan."