SINGAPORE — It will be a "bumpy road" to the nuclear negotiations with North Korea later this month, Defense Secretary Jim Mattis warned Sunday, telling his South Korean and Japanese counterparts they must maintain a strong defensive stance so the diplomats can negotiate from a position of strength.

"We can anticipate, at best, a bumpy road to the negotiations," Mattis said. "In this moment we are steadfastly committed to strengthening even further our defense cooperation as the best means for preserving the peace."

Mattis was speaking at the start of a meeting with South Korean Defense Minister Song Young-moo and Japanese Defense Minister Itsunori Onodera on the final day of the Shangri-La Dialogue security conference.

Plans are moving forward for an unprecedented summit between President Donald Trump and North Korean leader Kim Jong Un after Trump declared the summit back on following a historic and lengthy Oval Office meeting Friday with Kim Yong Chol, who is the top deputy to North Korean leader Kim Jong Un.

"We'll be meeting on June 12 in Singapore," Trump told reporters at the White House. But he also said that it would be hard to reach a deal from a single meeting in less than two weeks.

It was the latest dramatic twist in the high stakes diplomacy aimed at ending Pyongyang's nuclear weapons program.

Seeking to address concerns the U.S. may be rushing to strike a breakthrough, Mattis repeated the U.S. position that North Korea will only receive relief from U.N. national security sanctions when it demonstrates "verifiable and irreversible steps" to denuclearization.

He said allies must remain vigilant.

Through an interpreter, South Korean Defense Minister Song said that this is a great turning point as North Korea takes its first steps toward denuclearization.

"Of course, given North Korea's past, we must be cautious in approaching this," he added that some of North Korea's recent measures "give us reasons to be positive and one can be cautiously optimistic as we move forward."

Japanese Defense Minister Onodera said that while the solution to the North Korean nuclear crisis must be diplomatic, the defense cooperation among the United States and its Asian allies was key to bringing it about.

"Japan, Korea and the U.S. continue to agree that pressure is needed to be applied on North Korea," Onodera told reporters after the meeting.

Despite a long-standing security alliance between the United States and Japan, some people in Japan worry that the United States may cut a deal to protect its cities from nuclear attack by the North, while leaving Japan vulnerable.