Denuclearization is sure to top the agenda.

In a related vein, the two sides could also attempt to declare a formal end to the 1950-53 Korean War as a security assurance for the regime and may also discuss the issue of North Korea's alleged human rights abuses.

What kept the two countries' negotiators talking late into the night Monday, however, was likely the gap in their definition of denuclearization, and what that would entail.

The US seeks the complete, verifiable and irreversible dismantlement of the North's nuclear weapons program, whereas the North advocates a broader definition that also includes the removal of the US nuclear umbrella over South Korea and Japan.

"Meetings between staffs and representatives are going well and quickly ... but in the end, that doesn't matter," Trump tweeted.

"We will all know soon whether or not a real deal, unlike those of the past, can happen!"

US Secretary of State Mike Pompeo said Monday that the US goal has not changed. But he added that the US is ready to offer security assurances that are "different, unique" compared with those it has been willing to provide previously.

"The ultimate objective we seek from diplomacy with North Korea has not changed: a complete and verifiable and irreversible denuclearization of the Korean Peninsula is the only outcome that the United States will accept," he told reporters here. "But I am very optimistic that we will have a successful outcome from tomorrow's meeting between these two leaders."

Trump was also upbeat about the meeting he has said he has prepared for all his life.

"I just think it's going to work out very nicely," he said at a luncheon with Singapore's Prime Minister Lee Hsien Loong on Monday.

The first sit-down at the Capella Hotel will be joined by only the two leaders' respective translators, according to the White House.

That will be followed by an expanded bilateral meeting joined on the US side by Secretary of State Mike Pompeo, Chief of Staff John Kelly and National Security Advisor John Bolton.

Their final meeting, according to the White House, will be a working lunch attended by White House press secretary Sarah Sanders; Amb. Sung Kim, who has led substantive talks with the North going into the summit; and Matt Pottinger, senior director for Asia on the White House National Security Council.

The North Korean delegations have yet to be disclosed.

Ruling out the possibility of the talks extending into a second day, Trump will then hold a press conference before departing for the US at 7 p.m.

The on-again, off-again summit was reinstated on June 1 after an envoy of Kim's brought Trump a personal letter from the North Korean leader. Trump had called off the meeting the previous week, citing "open hostility" from the regime.

And in the weeks before that, North Korea threatened to pull out of the meeting over suggestions from Bolton and US Vice President Mike Pence that the regime would ultimately meet the same fate as Libya's collapsed government if it refused to denuclearize.

Trump in March accepted Kim's invitation to meet, a decision that took even his aides by surprise.

Behind that was Moon's push to improve ties with the North amid escalating fears of war prompted by an exchange of threats and personal insults between Trump and Kim over North Korea's testing of nuclear weapons and ballistic missiles last year. (Yonhap)