US President Donald Trump will hold a second summit with North Korean leader Kim Jong Un in late February but will maintain economic sanctions on Pyongyang, the White House said on Friday after Trump met Pyongyang's top nuclear negotiator.

The announcement came amid a diplomatic flurry in Washington surrounding the visit of Kim Yong Chol, a hardline former spy chief, and marked a sign of movement in a denuclearisation effort that has stalled since a landmark meeting between Trump and the North Korean leader in Singapore on June 12 last year.

"President Donald Trump met with Kim Yong Chol for an hour and a half to discuss denuclearisation and a second summit, which will take place near the end of February," White House spokeswoman Sarah Sanders said. She said a location would be announced later.

The summit was announced even though there has been no indication of any narrowing of differences over US demands that North Korea abandon a nuclear weapons programme that threatens the US and Pyongyang's demands for a lifting of punishing sanctions.

Sanders said that Trump's talks with the North Korean envoy were productive but added that the United States "is going to continue to keep pressure and sanctions on North Korea".

South Korea's presidential office said it expected the upcoming summit to be a "turning point to lay the firm foundation for lasting peace on the Korean peninsula".

South Korea will work with the US and other countries to "achieve concrete and practical results towards complete denuclearisation and a lasting peace regime through the North Korea-US summit...," presidential spokesperson Kim Eui-kyeom said in a statement on Saturday.

South Korea will also expand inter-Korean dialogue to help a successful meeting between Trump and Kim, he added.

The first summit produced a vague commitment by Kim Jong Un to work towards the denuclearisation of the Korean peninsula, but he has yet to take what Washington sees as concrete steps in that direction.

Nevertheless, both he and Trump had expressed an interest in arranging a second meeting, something some US-based analysts see as premature.

Critics of US efforts say the first summit only boosted Kim's international stature without much to show for it, and some believe Trump may see a second meeting as a way of distracting from his domestic troubles.

Trump declared just after the Singapore meeting that the nuclear threat posed by North Korea was over. But hours before Kim Yong Chol's arrival on Thursday, the US president unveiled a revamped US missile defence strategy that singled out the country as an ongoing and "extraordinary threat".

Harry Kazianis, an analyst at the conservative Washington-based Center for the National Interest, called the agreement to hold another summit positive.

But he added: “Both nations must now show at least some tangible benefits from their diplomatic efforts during a second summit, or risk their efforts being panned as nothing more than reality TV."

Communist-ruled Vietnam, which has good relations with both the United States and North Korea, has been widely touted as the most likely site of the next summit.

There has also been speculation about other possible venues, including Bangkok, Hawaii, or a return to Singapore.

Trump seeking ‘win’?

A senior US administration official said there was an exchange of letters for the leaders during the White House meeting, but gave no details of their content.

On his last visit to Washington in June, Kim Yong Chol delivered a letter from Kim Jong Un to Trump that helped overcome obstacles ahead of the summit in Singapore.

Kim Yong Chol, regarded as a member of Kim Jong Un's inner circle, also had talks with Secretary of State Mike Pompeo and US special representative on North Korea, Stephen Biegun.

A State Department statement said that after Pompeo and Kim met, the two sides had "a productive first meeting at the working level", and that Biegun would travel to Sweden at the weekend to attend an international conference.

The conference is also being attended by North Korean Vice-Foreign Minister Choe Son Hui. Washington has been keen to set up talks between Biegun and Choe but North Korea has resisted, apparently wanting to keep exchanges high-level.

South Korea's nuclear negotiator, Lee Do-hoon, also would attend the conference, the country's foreign ministry said, although it gave no indication of whether he would meet with Choe and Biegun.

US-based analysts said that North Korea, which has developed missiles and nuclear weapons in defiance of UN Security Council resolutions, would likely be seeking a clearer message from the Trump administration on any concessions it may be willing to make.

"The North Koreans need a real indication of what the US is willing to put on the table," said Jenny Town, a North Korea expert at 38 North, a Washington-based think tank.

South Korean Foreign Minister Kang Kyung-wha said on Wednesday that if North Korea took concrete steps toward abandoning its weapons programmes, Washington could offer a formal end to the 1950-53 Korean War, humanitarian aid or a permanent channel for bilateral dialogue.

Victor Cha, a former White House adviser on Asia under President George W. Bush, suggested that Trump may be so desperate for a policy "win" that he could be vulnerable to a bad deal with North Korea.

"I worry that the timing works to North Korea's benefit,” Cha said, citing pressures on Trump such as the partial US government shutdown and the ongoing investigation into alleged Russian ties to Trump's 2016 presidential campaign.

Earlier this month, Trump defended the stuttering progress on North Korea by saying that Pyongyang had stopped missile and bomb testing and if it had not been for his administration "you'd be having a nice big fat war in Asia".

- Reuters