US president Donald Trump plans to depart early from his unprecedented summit with Kim Jong Un, after declaring that nuclear talks with North Korea have moved "more quickly than expected".

Mr Trump had been scheduled to fly back to Washington on Wednesday morning after spending Tuesday with the North Korean leader in Singapore.

But on the eve of the summit, the US leader altered his timetable, opting to return almost 15 hours earlier than scheduled.

The White House said: "The discussions between the United States and North Korea are ongoing and have moved more quickly than expected."

It is not immediately clear what specific progress, if any, had been made in preliminary discussions between US and North Korean officials in the run-up to Tuesday's summit.

In fact, only hours before the White House announcement, US secretary of state Mike Pompeo had seemed to lower expectations for the meeting, which Mr Trump had earlier predicted could potentially yield an on-the-spot deal to end the Korean War.

"We are hopeful this summit will have set the conditions for future successful talks," Mr Pompeo said.

The summit - the first ever between a sitting American president and North Korea's leader - would have seen the two leaders greeting each other, before a one-on-one meeting which a US official said could last up to two hours, with only translators joining them.

The White House said the day-long summit would also include a working lunch and a larger meeting involving aides to both leaders.

On the US side, Mr Trump was to be joined by Mr Pompeo, his chief of staff John Kelly, national security adviser John Bolton and US ambassador to the Philippines Sung Kim, along with a few others.

Before flying home, Mr Trump planned to speak to reporters in Singapore after concluding the summit, the White House said.

The last-minute change of schedule came as both sides finalised preparations for the meeting.

Mr Trump has forecast a "nice" outcome to the talks, while Mr Kim spent the day out of view.