US President Donald Trump says a planned summit with Kim Jong-un is back on for June 12 in Singapore, after meeting with a senior envoy from Pyongyang who delivered a letter from the North Korean leader.

Key points: A top North Korean official met Mr Trump in the White House to deliver a letter from Kim Jong-un

A top North Korean official met Mr Trump in the White House to deliver a letter from Kim Jong-un Mr Trump said the June 12 meeting would be "a beginning"

Mr Trump said the June 12 meeting would be "a beginning" He said North Korea was committed to denuclearisation, and would be transformed under Kim Jong-un

Mr Trump said after an Oval Office meeting with North Korea's Kim Yong-chol that he would be making a mistake not to go forward with the on-again, off-again nuclear summit in Singapore.

He said his meeting with the most senior North Korean to visit the White House in 18 years lasted longer than expected, and "went very well".

"I think it's probably going to be a very successful, ultimately a successful process," he told reporters.

However, Mr Trump appeared to significantly lower expectations for the outcome of the historic summit, frequently describing it as the start of a process and not the place where the two leaders were likely to sign any agreement. He said a number of summits might be required.

"Frankly, I said, 'Take your time'," Mr Trump said.

Kim Yong-chol, a close aide to the North Korean leader, is the highest level figure from the secretive state to hold talks at the White House since a senior envoy visited former president Bill Clinton in 2000.

He was greeted at the White House by chief of staff John Kelly and then whisked into the Oval Office.

After the meeting, Mr Trump said Kim Jong-un was committed to denuclearisation, and that the letter was "a very nice letter, a very interesting letter".

"I look forward to the day when I can take the sanctions off of North Korea," Mr Trump said.

Kim Yong-chol presented Donald Trump with a "very interesting letter" from North Korean leader Kim Jung-un. ( Supplied: White House/Shealah Craighead )

'Our two countries face a pivotal moment'

Mr Trump said he believed transformation of North Korea could take place with Kim Jong-un as leader.

He added he did not believe the US would spend a lot of money in aid to North Korea, but South Korea would be responsible for that.

Kim Yong-chol's arrival in Washington came a day after Secretary of State Mike Pompeo declared that he was confident negotiations with Pyongyang were "moving in the right direction".

"Our two countries face a pivotal moment in our relationship, and it would be nothing short of tragic to let this opportunity go to waste," Mr Pompeo said in New York after meeting with Kim Yong-chol.

Mr Pompeo declared he was confident negotiations with Pyongyang were on the right track. ( AP: Andrew Harnik )

The United States previously blacklisted Kim Yong-chol, who once served as a bodyguard to the current leader's father Kim Jong-il, from coming to the United States because of his role in North Korea's military establishment.

After exchanging threats and insults since Mr Trump became President last year, the United States and North Korea have been trying to set up a summit — originally planned for Singapore on June 12 — between their leaders, at which Mr Trump wants to pressure Pyongyang into giving up its nuclear weapons.

The leadership in Pyongyang is believed to regard nuclear weapons as crucial to its survival and has rejected unilaterally disarming.

In Seoul, US negotiators on Friday expressed optimism after meeting their North Korean counterparts for preparatory talks at Panmunjom, on the fortified border between the two Koreas.

Wall Street took word that the summit was back on in stride. The S&P 500 added slightly to what were already healthy gains, then slipped back a few points to remain up 0.9 per cent in afternoon trade. The US dollar index added modestly to a gain.

Reuters/AP