North Korean leader Kim Jong-un visited tourist sites on Singapore's waterfront hours before his historic summit with Donald Trump, as the US President took to Twitter to hit out at the "the haters and losers" who "called him wrong from the beginning".

Key points: Kim Jong-un left his hotel for the first time since Sunday for sightseeing

Kim Jong-un left his hotel for the first time since Sunday for sightseeing Donald Trump is said to be 'optimistic' about the summit with Mr Kim

Donald Trump is said to be 'optimistic' about the summit with Mr Kim Meeting scheduled to start at 1100am (AEST)

The two leaders will make history when they meet later today, holding one-on-one talks before a working lunch with aides.

Ahead of the meeting Mr Kim's first stop was to visit the Gardens by the Bay, Singapore's famous waterfront park which boasts the largest glass greenhouse and tallest indoor, man-made waterfall in the world.

Police and onlookers were also seen gathering outside the Marina Bay Sands hotel, which resembles a ship perched on three columns at the mouth of the Singapore River.

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Singapore Foreign Minister Vivian Balakrishnan, who accompanied Mr Kim on his tour of the sites, posted a selfie to Facebook with the North Korean leader and Singapore Education Minister Ong Ye Kung.

According to South Korean media, Mr Kim was joined on his tour by his sister Kim Yo-jong and other top deputies.

The totalitarian leader, whose family has ruled North Korea with an iron fist for three generations, earlier met Singapore's Prime Minister Lee Hsien Loong soon after his arrival on Sunday in the city-state.

The Swiss-educated Mr Kim, who is believed to be 34, has not left his country since taking office in 2011 other than to visit China and the South Korean side of the border Demilitarised Zone, which separates the two Koreas.

Given North Korea's "utterly appalling" human rights record, seeing Mr Kim walking around Singapore was "rather uncomfortable for everybody", Foreign Minister Julie Bishop said.

"This is one of the concerns we have always had, that Kim Jong-un should be directing his efforts and energy and scarce resources to the betterment of his people rather than being a danger to our region and the globe."

Trump berates 'haters and losers'

United States President Donald Trump forecast a "nice" outcome for his meeting with Mr Kim.

He tweeted that although meetings between staff and representatives were going well, "in the end that doesn't matter".

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

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"The fact that I am having a meeting is a major loss for the US, say the haters & losers. We have our hostages, testing, research and all missle [sic] launches have stoped [sic], and these pundits, who have called me wrong from the beginning, have nothing else they can say! We will be fine!"

Both sides are finalising preparations for today's meeting, which is due to kick off at 9:00am local time (11:00am AEST).

The leaders are expected to talk one-on-one, with translators only, before heading into a larger meeting.

In a statement, the White House said Mr Trump would speak to the media before departing for the US in the evening.

Secretary of State Mike Pompeo said Monday's preparatory talks between the two nations had moved quite rapidly.

"We anticipate that they will come to their logical conclusion even more quickly than we anticipated," he said, calling the summit "an unprecedented opportunity to change the trajectory of our relationship and bring peace and prosperity" to North Korea.

However, he played down the possibility of a quick breakthrough and said the summit 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, Mr Pompeo said.

"If diplomacy does not move in the right direction … those measures will increase," he said.

Mr Pompeo said Mr Trump was "fully prepared" for the talks, and was optimistic that the outcome would be "successful".

The summit will be the first between a North Korean leader and a sitting American president.

Sorry, this video has expired Kim Jong-un meets Singapore's Prime Minister ahead of the historic summit. (Image: AP/Wong Maye-E)

'Excitement in the air'

The sense of anticipation was palpable as Mr Trump headed to meet with Singapore's PM on Monday, with people lining the streets waving mobile phones.

Although gaps remain over what denuclearisation would entail, Mr Trump sounded a positive note at the lunch meeting, telling Mr Lee, "we've got a very interesting meeting in particular tomorrow, and I think things can work out very nicely".

Mr Trump had earlier tweeted about the "excitement in the air".

A woman takes photos next to policemen standing guard outside St Regis hotel in Singapore. ( Reuters: Athit Perawongmetha )

Mr Trump and Mr Kim arrived in Singapore on Sunday, both staying at luxurious and heavily guarded hotels less than 1 kilometre apart, with Mr Trump at the Shangri-La Hotel and Mr Kim at the St Regis Hotel.

"The entire world is watching the historic summit between (North Korea) and the United States of America," Mr Kim told Mr Lee through an interpreter when they met on Sunday.

Sorry, this video has expired Donald Trump and Kim Jong-un arrive in Singapore ahead of historic summit

Adding to the excitement, former NBA basketball star Dennis Rodman arrived at Changi airport late last night.

He does not have any official place in the talks and he told reporters he was not sure if he would meet with Mr Kim in Singapore.

"I'm just happy to be a part of it because I think I deserve it, I think I brought awareness on a lot of things around the world, and I think North Korea has given a lot of people the opportunity to do this conference now. And I hope it's a success," Mr Rodman said.

Last week, Mr Rodman said he would "give whatever support is needed" to his "friends" Mr Trump and Mr Kim.

He is one of the few westerners to have met the North Korean leader on visits to the capital city Pyongyang and even sang Happy Birthday to Mr Kim during a 2014 visit.

Former NBA basketball player Dennis Rodman has "hung out" with Kim Jong-un on several occasions. ( AP: Wong Maye-E )

AP/Reuters