Mattis noted that North Korean leader Kim Jong Un committed to doing so in his June 12 meeting with President Donald Trump on denuclearizing the Korean peninsula.

He said that the United Nations Command in South Korea is standing by to receive the remains.

"We simply are standing by for whenever the diplomatic activities are done," he told journalists.

"We are optimistic that it will begin," he added, because Kim agreed to it.

More than 35,000 Americans were killed on the Korean Peninsula during the war, which ended in an armistice with no peace treaty.

Among them, 7,700 are still considered missing, including 5,300 in North Korea alone, according to the Pentagon.

The Pentagon says Pyongyang has indicated several times that they have as many as 200 sets of remains that could be those of US soldiers who died in the war.

But Pentagon officials cautioned that it is unclear just how much North Korea is preparing to hand over.

The remains would have to go through an initial check in South Korea to confirm they are human remains.

Then they will be transferred to Hawaii where military forensic scientists will try to identify them.

Between 1990 and 2005, 229 sets of remains from the North were repatriated, but those operations were suspended when ties between the two countries deteriorated over Pyongyang's continuing effort to develop nuclear weapons.