"It represents in our view a historic opportunity for Chairman Kim (Jong Un) to follow through his commitment that he made to President (Donald) Trump," State Department spokeswoman Heather Nauert said.

"We hope to see a meaningful verifiable step toward the denuclearization of North Korea," she added about the third meeting between Kim and the South's Moon Jae-in.

During their June summit in Singapore, Kim had made a vague promise to Trump about aiming for complete denuclearization of the Korean peninsula.

The specifics and deadlines were due to be hashed out at a follow-up meeting that has yet to be set but that Seoul hopes to relaunch during the inter-Korean summit running through Thursday.

Asked about possible progress for Kim and Moon to formally declare an end to the Korean War -- which only concluded with an armistice in 1953 -- the American spokeswoman refused to specify whether Washington would join the statement and under what conditions.

"We will continue to consult very closely, carefully and regularly with the South Koreans as they continue to have their meetings in North Korea," Nauert said.

Observers worry that Trump could grant Kim a formal peace declaration without obtaining anything concrete in exchange.