It is also unclear whether USA officials will raise human rights issues in meetings with the North Koreans.
"It's the case in each of those two countries there are only two people that can make decisions of this magnitude and those two people are going to be sitting in the room together tomorrow", said Pompeo.
The White House also announced Trump would be leaving Singapore earlier than previously expected and holding a media availability ahead of his departure.
In the hours before the summit began Trump expressed optimism about prospects for the first-ever meeting of sitting U.S. and North Korean leaders, while U.S. Secretary of State Mike Pompeo injected a note of caution whether Kim would prove to be honest about his willingness to denuclearise.
Trump and Kim are staying in separate hotels less that a kilometre apart in the famous Orchard Road area of Singapore, dotted with high-rise luxury apartment blocks, offices and glittering shopping malls.
Trump said Tuesday after meeting Kim that he's feeling "really great".
Trump and his advisers have said the USA would accept nothing less than full denuclearization before sanctions against the North are lifted, and before North Korea can be fully welcomed into the fold of the broader diplomatic community.
Trump and Kim will be meeting at the Capella Hotel on the island resort of Sentosa and this will be the first meeting between a sitting USA president and the leader of North Korea.
The unraveling of the Canada summit meeting increases the North Korean leader's incentive to "up his asks and limit his compromises and for Trump to do the opposite", Richard Haass, the president of the Council on Foreign Relations, said on Twitter, adding, "Hardly the ideal context". Mr Trump says signing a peace agreement would probably be "the easy part".
North Korean vice minister of foreign affairs Choi Sun-hee (third from left) arrives at the Ritz Carlton Hotel in Singapore to continue talks with the American ambassador to the Philippines, Sung Kim.
But that tweet arrived after the White House ruled out any role for the high-profile retired National Basketball Association player in the hotly-anticipated talks with the North Korean leader, with whom Rodman has struck up an unlikely friendship.
Trump has said he'll know within minutes whether a deal can be made.
The results from previous summits and peace talks show that getting North Korea to seriously commit to denuclearization at any level is an vast challenge. The meeting was initially meant to rid North Korea of its nuclear weapons, but the talks have been portrayed by Trump in recent days more as a get-to-know-you session.
Here's how the day is scheduled to go: Trump and Kim will meet at 9am local time and sit down for a one-on-one discussion (with translators, but no advisers present) at 9.15. Meanwhile, Mr Kim has called Mr Trump "mentally deranged" and a "dotard". On a larger level, the astronomical cost of the nuclear weapons program contributes to the massive poverty in North Korea, among the world's poorest countries.
In the past two months, he has met twice with South Korean President Moon Jae-in and twice with China's leader Xi Jinping.
Laboriously negotiated past nuclear deals, considered breakthroughs at the time, broke down on North Korea's extreme sensitivity to allowing in outsiders to look at whether they're dismantling their nuclear facilities, many of which are thought to be hidden.