WASHINGTON/SEOUL -- The United Nations Command in South Korea will handle North Korea's return of the remains of American troops killed in the 1950-53 Korean War, US Defense Secretary Jim Mattis has said.

The repatriation was part of an agreement reached by US President Donald Trump and North Korean leader Kim Jong-un at their historic summit on June 12.

US Forces Korea said over the weekend that it had moved 100 wooden coffins to the inter-Korean border to prepare for the remains' delivery, with about 200 expected to be returned.

"The United Nations Command in ROK, in the Republic of Korea, is prepared, now, to receive those remains," Mattis told reporters Sunday en route to Alaska, according to an official transcript.

From Alaska, the US defense chief is set to travel to China, South Korea and then Japan for consultations with his counterparts.

"They have staged appropriate logistics materials, and we simply are standing by for whenever the diplomatic activities are done. And we're optimistic that it will begin because that was an agreement coming out of Singapore," Mattis said.

The UNC oversees the cease-fire that ended the Korean War.

Mattis explained that the UNC was chosen to receive the remains, not the South Korean or US militaries alone, because all 21 nations that fought under the UN flag alongside South Korea and the US lost troops in the conflict.

Once the remains cross the inter-Korean border, they will be moved to the US air base in Osan, south of Seoul.

"There, they will be checked to make sure that they appear to be what we think they are, just to make sure that they're probably from either western countries or other countries that were sending states," Mattis said, adding that the process will take a couple days or a week.

The remains will then be sent to Hickam Air Force Base in Hawaii for forensic identification.

Mattis is scheduled to arrive in Seoul on Thursday for talks with South Korean Defense Minister Song Young-moo.

"Alongside the People's Republic of China and Japan, the United States and the Republic of Korea have a common goal," he said.

"That's the complete, irreversible and verifiable denuclearization of the Korean Peninsula, and this will include our agreed-upon reduction to larger-scale exercises, and between Minister (Song) and I, we'll consult on the way ahead."

Mattis was referring to Trump's decision to suspend joint military exercises between the allies while negotiations to dismantle the North's nuclear weapons program are under way.

Early last week Seoul and Washington announced the suspension of a major military exercise slated for August. The Pentagon then added two marine exchange program training exercises to the list Friday.

"We considered both to be consistent with what the two heads of state had come out with," Mattis said.

The defense chiefs of South Korea and the US are set to hold talks Thursday.

Seoul's defense ministry said that Song and Mattis will discuss joint efforts to enforce recent summit agreements with the North and key alliance issues, such as the envisioned transfer of wartime operational control.

They are also expected to consult over the allies' decisions to suspend their military exercises, including the Ulchi Freedom Guardian initially scheduled for August.

"Currently, the two countries share the understanding that they would suspend the large-scale or aggressive combined exercises for the time being while carrying out humanitarian search and rescue drills normally," a government official said on condition of anonymity. (Yonhap)