At the heart of the summit is whether, and in what detail, Trump and Kim will agree to dismantle the North's nuclear weapons program in exchange for security guarantees for the regime.

Trump has said he and Kim could also declare a formal end to the 1950-53 Korean War that ended in an armistice, not a peace treaty.

"I don't want to speculate on the timing or what may or may not happen," Nauert said. "I know that that's something that the president and this administration certainly supports."

She refused to discuss the possibility that South Korean President Moon Jae-in will join Trump and Kim in Singapore for such a declaration.

"I can tell you we constantly are in conversation with the government of South Korea as we are with Japan as well," the spokeswoman said. "A trip to Singapore to my knowledge has not come up but I'd have to refer you back to the government of South Korea."

US Secretary of State Mike Pompeo met with Singaporean Foreign Minister Vivian Balakrishnan in Washington earlier in the day and "expressed appreciation" for the city-state's willingness to host the summit, according to an official readout.

Pompeo has also been receiving briefings, sometimes multiple times a day, from Amb. Sung Kim, who has been leading a US delegation to pre-summit negotiations with North Korea on the inter-Korean border, Nauert said. (Yonhap)