US President Donald Trump says his historic summit with North Korean leader Kim Jong Un in Singapore could "work out very nicely" as officials from both countries met to narrow differences on how to end a nuclear stand-off on the Korean peninsula.
US Secretary of State Mike Pompeo says Monday's preparatory talks were moving quite rapidly and "and we anticipate that they will come to their logical conclusion even more quickly than we anticipated."
The summit provides "an unprecedented opportunity to change the trajectory of our relationship and bring peace and prosperity" to North Korea, Pompeo told a news conference on the summit's eve.
However, he played down the possibility of a quick breakthrough and said the summit, which gets underway on Tuesday at 9am (11am AEST) should set the framework for "the hard work that will follow", insisting that North Korea had to move toward complete, verifiable and irreversible denuclearisation.
Sanctions on North Korea would remain in place until that had happened, Pompeo said. "If diplomacy does not move in the right direction...those measures will increase."
Kim and Trump arrived in tropical Singapore on Sunday for the first ever face-to-face meeting by leaders of two countries that have been enemies since the 1950-1953 Korean War.
Although gaps remain over what denuclearisation would entail, Trump sounded a positive note in a lunch meeting with Singapore's Prime Minister Lee Hsien Loong.
"We've got a very interesting meeting ... tomorrow, and I just think it's going to work out very nicely," Trump said.
Lobster bisque, beef tenderloin and ice cream were on the lunch menu, and there was also an early birthday cake for Trump, who turns 72 on Thursday.
Kim remained ensconced in the heavily guarded St Regis Hotel, where he is staying. There was also no sign of his sister, Kim Yo Jong, who has accompanied him to Singapore.
Some people were grumbling in the wealthy city-state because of traffic jams caused by the summit and the cost of hosting two leaders with massive security needs. Lee has said the summit would cost Singapore about S$20 million ($A19m), more than half of which would go on security.
"Thanks PM Lee for spending $20 million of taxpayers money, which can ... help a lot of needy families in Singapore to survive," posted one Facebook user. Others complained about the traffic jams in downtown Singapore.
Lee said the cost was worthwhile.
"It is our contribution to an international endeavour which is in our profound interest," he told reporters on Sunday.
Trump and Kim are staying in separate hotels in the famous Orchard Road area of Singapore, dotted with high-rise luxury apartment blocks, offices and glittering shopping malls. Traffic was held up in the steamy midday sun and scores of bystanders were penned in by police when Trump went to meet Lee.
Similar scenes were seen on Sunday when Kim and Trump arrived in the city, and when Kim went to meet Lee. Their hotels are cordoned off with heavy security.
Commenting for the first time on the summit, North Korea's state-run KCNA news agency earlier said the two sides would exchange "wide-ranging and profound views" to re-set relations. It heralded the summit as part of a "changed era".
Discussions would focus on "the issue of building a permanent and durable peace-keeping mechanism on the Korean peninsula, the issue of realising the denuclearisation of the Korean peninsula and other issues of mutual concern," KCNA said.
