The new preliminary talks are set to continue through Tuesday, and are being led by Sung Kim, a former USA ambassador to South Korea who has held nuclear program negotiations with North Korea in the past.
At the White House, Trump said negotiations over a potential June 12 summit with Kim that he had earlier canceled were "going along very well".
North Korea and the US appear to be back at work making preparations for their summit. "It will happen!" Trump tweeted at about 1am South African time.
Meanwhile, in the USA, the White House announced that a pre-advance group was already on its way to Singapore for preparation of logistics for the summit on May 27.
Sung Kim had been on standby in Seoul since last week. But China, one of North Korea's largest trade partners, drastically reduced its imports from North Korea this year, according to the Associated Press.
In fact, Japan's Kyodo News Agency reported Monday, citing USA officials, . that Washington is asking Pyongyang to first transfer the 20 nuclear warheads it is believed to have.to somewhere outside of its borders as soon as possible.
A USA official said Sung Kim, the former U.S. ambassador to South Korea, would lead an American delegation to meet North Korean officials at the border.
"The president was always clear that he was prepared to walk away from this meeting, and he has kept his word", the official said.
The report said meetings with Sum's group would continue through Tuesday at Unification House, in the northern part of the demilitarized zone.
The source said agreement to hold the talks was reached between Washington and Pyongyang "through a separate channel" from a surprise summit between Kim Jong-un and President Moon Jae-in on Saturday.More news: Hawaii's Kilauea volcano spews lava
"Chairman Kim and I have agreed that the June 12 summit should be held successfully, and that our quest for the Korean peninsula's denuclearisation and a perpetual peace regime should not be halted", Mr Moon said.
North Korea has rejected the US's demand for it to unilaterally abandon its nuclear weapons programme. He expressed frustration immediately afterward, calling the move "very regrettable". "But at the same time, he has healthy skepticism", the official added. Moon said North Korea was not convinced it could trust security guarantees from the United States.
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.
The leaders of North and South Korea had discussed a three-way summit on April 27 at their first meeting in Panmunjom.
Higher-level talks could also occur before the summit, the official said, including further discussions between Secretary of State Mike Pompeo and top North Korean officials, including possibly Kim.
"The goal here is to achieve maximum pressure", a senior administration official told The Journal last week.
China, South Korea and the United States all back denuclearisation of the Korean Peninsula, but differ on how to make that happen.
Libyan leader Muammar Gaddafi was deposed and killed by Nato-backed militants in 2011 after halting his nascent nuclear program. And that means the "Libya model" is indeed on the table. "He's going to run this the way he sees fit".
South Korea's Yonhap news said Tuesday it saw the name of Kim Yong Chol on the passengers' list for a flight Tuesday from Beijing to Washington.