President Donald Trump confirmed Sunday that U.S. negotiators have arrived in North Korea to make preparations for planned June 12 summit with dictator Kim Jong-un.

Trump tweeted about the diplomatic move hours after a report that a top U.S. diplomat had crossed the DMZ to begin preparations for a summit that was on, then off, then potentially on again.

'Our United States team has arrived in North Korea to make arrangements for the Summit between Kim Jong Un and myself,' Trump tweeted Sunday afternoon after spending four hours at his Virginia golf course.

'I truly believe North Korea has brilliant potential and will be a great economic and financial Nation one day. Kim Jong Un agrees with me on this. It will happen!' he wrote.

US Ambassador to the Philippines Sung Kim, who previously served as a nuclear negotiator with the North Koreans, has crossed into North Korean territory in advance of a possible summit

In the latest signal that the U.S. is determined to push ahead with a possible summit between President Trump and North Korean dictator Kim Jong-un in June, a seasoned U.S. diplomat crossed the DMZ Sunday to begin preparations the talks.

The move came just days after President Donald Trump announced he was withdrawing from the summit planned for June 12, only to reverse himself after the North Korean side came back with a surprisingly conciliatory response.

Sung Kim, the U.S. ambassador to the Philippines who has led nuclear talks with North Korea in the past, crossed into the hermetic country on Sunday in order to begin preparations, The Washington Post reported, citing a person familiar with the arrangements.

Sung Kim was born in South Korea and previously served as the U.S. ambassador to that country. President Barack Obama nominate the career diplomat to his top posts in both South Korea and the Philippines, and the Senate confirmed him to both.

Choe Son-hui, Head of the North America Department of North Korea's Foreign Ministry, issued a statement calling Vice President Mike Pence a 'political dummy' and slammed him for 'ignorant and stupid remarks'

Both Pyongyang and Washington are pressing ahead on plans for a meeting between the two leaders after Trump pulled out of the scheduled June 12 summit on Thursday, only to reconsider the decision the next day.

The movement by the U.S. side came as North Korean leader Kim Jong-un reaffirmed his commitment to meet with Trump. He provided the assurance to his South Korean counterpart after a spontaneous meeting in the DMZ following Trump's announcement he would pull out of the summit.

President Donald Trump has announced the summit with North Korea will take place as planned on June 12 in Singapore

Sung Kim crossed into North Korean territory with Allison Hooker, the Korea expert on the White House National Security Council.

They met with Choe Son Hui, the North Korean vice foreign minister, the Post reported.

It is she who issued a statement blasting Vice President Mike Pence for 'ignorant and stupid' remarks – an event that triggered Trump's letter announcing his decision to withdraw from talks.

Pentagon official Randall Schriver is also in Seoul currently, the Post said.

The meetings are expected to continue on Monday and Tuesday and are focused on the issue of North Korea´s nuclear weapons program, it said.

The latest action in the improvisational move toward a possible summit comes as the former head of the National Security Agency, Michael Hayden, said Trump hadn't 'done his homework' on North Korea's nukes.

'Kim Jong-un knows his program inside and out,” Hayden told ABC News 'This Week' on Sunday.

“I think he knows what he can concede and what it means and what he cannot concede,' Hayden said.

“I don't know that the president has done the kind of homework" said Hayden, who also served as CIA director.

Trump said Saturday after an event with reporters that things were going 'very well' in terms of the summit with North Korea.

'I think there's a lot of good will,' the president said.

LETS TALK: US Ambassador to the Philippines Sung Kim looks inside a Cessna-208B Grand Caravan Intelligence, Surveillance and Recoinnaissance (ISR) aircraft acquired through grant programmes by the the US government to the Philippine Air Force (PAF) during a formal turn-over ceremony at the PAF headquarters in Manila on July 27, 2017

Choe Son-hui, Head of the North America Department of North Korea's Foreign Ministry, issued a statement calling Vice President Mike Pence a 'political dummy' and slammed him for 'ignorant and stupid remarks.' Here she arrives at the 2017 Moscow Nonproliferation Conference at the Centre for Energy and Security Studies.

(L-R) Philippine military exercise director Lieutenant General Emmanuel Salamat, US Ambassador to the Philippines Sung Kim, Philippine Defense Secretary Delfin Lorenzana, Philippine military chief, General Carlito Galvez, and US military exercise director Lieutenant General Lawrence Nicholson link arms during the opening ceremony of the joint US-Philippines military exercise opening ceremony in Manila on May 7, 2018. Sung Kim has crossed into North Korea for talks

'People want to see if we can get the meeting and get this done. We are looking at June 12th in Singapore.'

Trump had cancelled the meeting on Thursday, blaming 'hostility' shown by a North Korean official who threatened nuclear war for the change, but appeared to have been calmed by apologetic communications from Pyongyang.

He explained his reasoning, and threatened a nuclear showdown with North Korea, through a letter sent to leader Kim Jong Un.

Kim Jong Un was scheduled to meet with Trump in Singapore for historical talks on denuclearization

'Based on the tremendous anger and open hostility displayed in your most recent statement, I feel it is inappropriate, at this time, to have this long-planned meeting,' he wrote.

'Therefore, please let this letter serve to represent that the Singapore summit, for the good of both parties, but to the detriment of the world, will not take place.'

South Korean President Moon Jae-in said earlier that he and North Korea's Kim agreed at a surprise meeting on Saturday that the possible North Korea-U.S. summit must be held, Moon told a news conference in Seoul.

The pair recommitted to their 'quest for the Korean peninsula's denuclearization and a perpetual peace regime'

Pictured: The letter President Trump sent Kim Jong Un after 'hostile' remarks from a North Korean official

Kim reaffirmed his commitment to "complete" denuclearization of the Korean Peninsula and to a planned meeting with Trump, Moon told a news conference in Seoul.

"Chairman Kim and I have agreed that the June 12 summit should be held successfully, and that our quest for the Korean peninsula's denuclearization and a perpetual peace regime should not be halted," Moon said.

The meeting was the latest dramatic turn in a week of diplomatic ups and downs surrounding the prospects for an unprecedented summit between the United States and North Korea, and the strongest sign yet that the leaders of the two Koreas are trying to keep the on-again off-again meeting on track.

While maintaining that Kim is committed to denuclearization, Moon acknowledged Pyongyang and Washington may have differing expectations of what that means and he urged both sides to hold working-level talks to resolve their differences.

A statement from North Korea's state news agency KCNA said Kim expressed "his fixed will" on the possibility of meeting Trump as previously planned.

Trump said on Saturday he was still looking at a June 12 date for a summit in Singapore and that talks were progressing very well.

"We're doing very well in terms of the summit with North Korea," Trump said at the White House. "It's moving along very nicely. So we're looking at June 12th in Singapore. That hasn't changed. So, we'll see what happens."

A White House team will leave as scheduled for Singapore this weekend to prepare for the possible summit, a White House spokeswoman said on Saturday.

Moon, who returned to Seoul on Thursday morning after meeting Trump in Washington in an effort to keep the U.S.-North Korea summit on track, said he delivered a message of Trump's "firm resolve" to end the hostile relationship with North Korea and pursue bilateral economic cooperation.

Trump said in a letter to Kim on Thursday he was cancelling the planned Singapore summit, citing North Korea's "open hostility."

The Trump administration has demanded that North Korea completely and irreversibly shut its nuclear weapons program. Kim and Trump's initial decision to meet followed months of war threats and insults between the leaders over the program.

American officials are skeptical that Kim will ever fully abandon his nuclear arsenal, and Moon said North Korea is not yet convinced it can trust security guarantees from the United States.

(Reporting by Hyonhee Shin in Seoul and Doina Chiacu in Washington Additional reporting by Josh Smith, Soyoung Kim, Choonsik Yoo and Jeongmin Kim in SEOUL, and Ben Blanchard and Elias Glenn in BEIJING Editing by Raju Gopalakrishnan, Paul Tait and Frances Kerry)