Pentagon official Randall Schriver was part of the U.S. team, the official said.
The officials are meeting through Tuesday attempting to get the June 12 summit in Singapore back in motion. "What's unclear for Chairman Kim, in my opinion, is not his willingness for denuclearisation, but whether he can certainly trust the USA saying that it'll end hostile relations and guarantee the security of his regime after his denuclearisation".
South Korea's President Moon Jae-in (R) hugs North Korea's leader Kim Jong-un after their second summit at the north side of the truce village of Panmunjom in the Demilitarized Zone (DMZ) on May 26, 2018.
"I truly believe North Korea has brilliant potential and will be a great economic and financial Nation one day", Trump said.
Events around the Korean peninsula have been moving at a dizzying pace over the last few days, with a summit between Donald Trump and Kim Jong-un that was on, then off, then apparently on again.
Sung Kim had been on standby in Seoul since last week.
The third team on a separate but complementary track to the other discussions was a CIA team Pompeo set up past year when he headed the spy agency.
The White House confirmed that an advance team of officials would leave for Singapore this weekend, as originally scheduled, to prepare for the possible summit. It was led by Joe Hagin, White House deputy chief of staff for operations.
According to The Washington Post, Sung Kim, a former United States ambassador to South Korea and former nuclear negotiator with the North, has been called in from his post as envoy to the Philippines to lead the preparations. Moon called the gathering a meaningful attempt to clear up "some difficulties in communication" as the two leaders shared warm words on the northern side of their border.
Kim may see the sit-down with Trump as necessary to easing pressure from crushing sanctions and to winning security assurances in a region surrounded by enemies. He was initially booked on an Air China flight to Washington but changed it to a Wednesday flight bound for NY, the agency said.
Keeping Summit Hopes Alive Suggests Kim Wants and Needs a Deal, Too
State Department officials would leave for Singapore on schedule this weekend to prepare for a possible summit there. On Thursday, President Trump cancelled the planned summit after a threatening statement issued by North Korea .
"A lot of people are working on it", Trump said the same day.
Moon said he told Kim that Trump has a "firm resolve" to end hostile relations with North Korea and initiate economic cooperation if Kim implements "complete denuclearization". This was portrayed as a sign of good faith, but as the Guardian newspaper recently reported, "North Korea's main nuclear test site has partially collapsed under the stress of multiple explosions, possibly rendering it unsafe for further testing".
General Kim Yong Chol landed at Beijing airport on Tuesday and will journey on to NY the following day after talks with Chinese officials, according to South Korea's Yonhap news agency, which cited diplomatic sources.
He also accompanied Kim Jong Un on both of his recent trips to China to meet President Xi Jinping and held talks with Secretary of State Mike Pompeo when he travelled to Pyongyang.
Timeline for denuclearization is expected to be a major sticking point in any negotiations between Trump and Kim.
Moon said Monday there could be more impromptu talks between the two Koreas in the lead-up to the summit.
"Because, if they don't have money, they can't launch missiles, they can't detonate nukes, and Kim Jong Un can't give gifts to senior regime members in order to buy loyalty, which means he has to give up his weapons".
It has long said it is open to eventually giving up its nuclear arsenal if the United States withdraws its troops from South Korea and ends its "nuclear umbrella" alliance with Seoul.