"We don't know. Kim might also want security assurances from China, Russia, and the United Nations Security Council", Kovrig said. "No president has ever put America in a position where the North Korean leadership thought that this was truly possible".
Washington is demanding the North give up its weapons in complete, verifiable and irreversible way (CVID), while Pyongyang has so far only made public pledges of a commitment to the denuclearisation of the peninsula-a term open to wide interpretation.
Meanwhile, the USA ambassador to the Philippines, Sung Kim, plans another round of meetings on Monday afternoon with North Korean officials as both sides continue to prepare for Tuesday's summit in Singapore.
In terms of security guarantees, Kim is eager to receive substantial compensation - such as a non-aggression pact, a declaration to end the Korean War, establishing diplomatic relations and easing up on economic sanctions - from Trump.
For his April meeting with South Korean President Moon Jae-in in the border village of Panmunjom - the first meeting between North and South Korean leaders in more than a decade - the North Koreans also dispatched a portable toilet for Kim.
Sung Kim, a former USA ambassador to South Korea who has been leading the U.S. in talks with North Korean Vice-Foreign Minister Choe Son Hui since last month, is said to be continuing the discussions with his counterpart in Singapore. The U.S. president turns 72 on June 14.
Offering a preview to reporters on the eve of the summit, Pompeo said it could provide "an unprecedented opportunity to change the trajectory of our relationship and bring peace and prosperity" to North Korea.
Another possibility from the summit is a deal to end the Korean War, which North Korea has long demanded, presumably, in part, to get United States troops off the Korean Peninsula and eventually pave the way for a North Korean-led unified Korea.
Mr Trump descended from Air Force One into the steamy Singapore night, greeting officials and declaring he felt "very good" before being whisked away to his hotel via a route lined with police and photo-snapping onlookers.
Trump said he expected the denuclearisation process to start "very, very quickly".
At the signing ceremony, Trump said he expected to "meet many times" in the future with Kim.
Trump also tweeted: "Meetings between staffs and representatives are going well and quickly. but in the end, that doesn't matter".
"We'll probably need another summit", Trump said during the news conference that lasted more than an hour.
Singapore is hosting Tuesday's historic summit between Trump and North Korea's Kim Jong Un to discuss the fate of North Korea's nuclear weapons program.
Iran says North Korea should be wary of negotiating with President Donald Trump following his withdrawal from the landmark 2015 nuclear agreement. Another Rodman trip to North Korea was sponsored by online betting company Paddy Power, but that firm severed ties with Rodman in 2013 after taking heat for associating with the North Korean regime.
During the meeting, the officials also celebrated Trump's birthday.
Kim spent the day out of view, but was reportedly going on an evening tour of attractions on the city's waterfront, a source familiar with the plan told Reuters.
Bandow noted that American companies could also turn to North Korea for low-priced manufacturing, which neighbors - like China and South Korea - already benefit from.