And until denuclearisation takes place, sanctions against North Korea would remain in place, he stressed.
Kim will be able to claim a huge propaganda bonus by sitting as an equal with the U.S. president, an accomplishment his father and grandfather sought but could never realise.
"I feel that Kim Jong Un wants to do something great for his people and he has that opportunity".
The issues on the table remain stubborn and complex for two countries that have been at odds for almost 70 years.More news: Bethesda Announces THE ELDER SCROLLS VI and Mobile Game at E3
The US was prepared to offer certainty to North Korea so they could be "comfortable that denuclearisation isn't something that ends badly for them".
"I think I'll know pretty quickly whether or not, in my opinion, something positive will happen", said Trump.
Trump acknowledged the difficulty of gleaning much information concerning Kim, who has scant experience on the global stage and about whom foreign intelligence agencies have struggled to gather much beyond basic biographical data. "Why would Kim. believe any commitments President Trump makes when he arbitrarily tears up an agreement with which the other party is complying?" said Antony Blinken, who was deputy secretary of state under Barack Obama.
"I'm very optimistic that we will have a successful outcome from our meetings tomorrow", he said.
Tuesday's Singapore meeting is the climax of the astonishing flurry of diplomacy on and around the Korean Peninsula this year, but critics charge that it risks being largely a triumph of style over substance. He noted the goodwill gesture of Pyongyang recently releasing "three hostages" - Americans who had been imprisoned in the country - and that was enough for him to proceed with the summit.More news: Kingdom Hearts 3 release date announced
As anticipation for his historic summit with North Korea has ratcheted up, salesman in chief Donald Trump has done something out of character: ratcheted expectations down.
On Monday Pyongyang's state-run newspaper, one of the only outlets North Koreans can freely read, appeared to be laying the groundwork for a new, normalized relationship with the US. "Obviously what has been done before hasn't worked".
Kim's summit with Trump has captured intense global attention after a turn to diplomacy in recent months after serious fears of war past year amid North Korean nuclear and missile tests.
Pompeo has disputed the notion that the president is moving the goalposts, telling reporters Thursday that Trump "has always understood that this was a process".
Earlier Monday, Trump similarly expressed optimism during a working lunch with Singapore's Prime Minister Lee Hsien Loong and aides to both leaders.
"I am on my way to Singapore where we have a chance to achieve a truly wonderful result for North Korea and the World", Trump wrote in a pair of tweets.More news: U.S. Expands China Health Alert Amid Mysterious Illness Reports
Secretary of State Mike Pompeo said Monday he is "unconcerned" about the diplomatic crisis caused by President Trump's adversarial trade posture with America's closest western allies and his extraordinary insults directed at the Canadian prime minister.