A top North Korean official widely known as leader Kim Jong Un’s right-hand man has arrived in Beijing, en route to the United States for a meeting with top U.S. officials, a report said Tuesday, as Washington and Pyongyang continue to lay the groundwork for a historic leaders’ summit.

The trip to the U.S. by former spy chief and senior official Kim Yong Chol is the highest-profile visit in nearly 18 years as the two sides work to bridge gaps over Pyongyang’s nuclear weapons program ahead of a planned summit between North Korean leader Kim Jong Un and U.S. President Donald Trump that had been scheduled for June 12 in Singapore.

Kim Yong Chol arrived in Beijing on Tuesday for talks with Chinese officials, South Korea’s Yonhap news agency said, and he was due to depart for New York the following day. In a tweet, Trump confirmed that Kim was heading for the United States.

“We have put a great team together for our talks with North Korea. Meetings are currently taking place concerning Summit, and more,” Trump wrote. “Solid response to my letter, thank you!”

South Korean and Japanese media reports said that Choe Kang Il, deputy director-general for North American affairs at the North Korean Foreign Ministry was also among the officials who arrived from Pyongyang, and could travel on to New York.

Kim Yong Chol is officially vice chairman of the central committee of North Korea’s all-powerful Workers’ Party. But in reality, as the onetime head of the Reconnaissance General Bureau, the North’s intelligence agency, he is the regime’s spy chief and a top adviser to the supreme leader.

Kim, who is known to be well-versed in denuclearization and security issues, is subject to sanctions by the U.S. Treasury Department for his involvement in the North’s nuclear program and other illicit activities. Because of this, he is banned from visiting the United States normally, indicating a waiver would be needed to enter.

He has also faced criticism for his alleged role in a series of North Korean provocations, including the 2010 sinking of the South Korean Navy corvette the Cheonan, which killed 46 sailors.

His trip to the U.S. will make him the highest-level North Korean official to visit Washington since 2000, when then-North Korean leader Kim Jong Il sent Gen. Jo Myong Rok, the second most powerful person in the country at the time, to meet Clinton administration officials at the White House as a special envoy.

News of the latest trip came after a team of veteran U.S. negotiators involved in preparatory discussions for the Trump-Kim summit met with top North Korean officials at the truce village of Panmunjom that divides the two Koreas on Sunday.

The team was being led by Sung Kim, a former U.S. ambassador to South Korea and onetime nuclear negotiator with the North, who had been called in from his posting as envoy to the Philippines to lead the meetings. Media reports Tuesday said the team left a hotel in Seoul amid speculation that they would be resuming the talks.

The White House also sent a logistical group to Singapore on Sunday to prepare in the event the summit is held there on June 12. It is led by Joe Hagin, White House deputy chief of staff for operations. NHK reported Monday that a U.S. government aircraft carrying that delegation had departed from Yokota Air Base on the outskirts of Tokyo en route for Singapore. In a separate report the same day, it said that Kim Chang Son, Kim Jong Un’s de facto chief of staff, had arrived in the city-state Monday, showing footage of him at the airport escorted by three bodyguards.

Trump withdrew from the on-again-off-again summit last Thursday, after several turbulent days of diplomatic brinkmanship that culminated in an abrupt letter written by the president to Kim Jong Un citing Pyongyang’s “tremendous anger and open hostility.” He has since said the meeting could still happen.

Observers say the trip by Kim Yong Chol could be a reciprocal visit after U.S. Secretary of State Mike Pompeo twice traveled to Pyongyang in recent weeks. Pompeo held talks in the North Korean capital with Kim Jong Un, who was accompanied by his spy chief.

The mission to the United States by Kim Yong Chol, who is in charge of inter-Korean relations and has been a fixture in meetings with South Korean officials — including a summit Saturday with President Moon Jae-in — would further signal that preparations for the U.S.-North Korea summit are continuing to move forward.

Andrew O’Neil, an expert on North Korea and a professor at Griffith University in Australia, called the visit a “significant development.”

“Kim Yong Chol is clearly the single most-trusted North Korean official outside Kim Jong Un’s immediate family, and … has been a conspicuous presence in the season of summitry with South Korea and China.”

Perhaps more importantly in regards to any U.S. visit, he has also served as “the key interlocutor for Pompeo” in the top American diplomat’s capacities as both CIA chief and secretary of state.

“Kim Yong Chol’s hands-on role is a prerequisite for detailed North Korean engagement in a U.S.-DPRK summit, so if confirmed, his visit to the U.S. is a major development,” O’Neil said, using the acronym for the North’s formal name, the Democratic People’s Republic of Korea.

He “will be authorized to speak on behalf of Kim Jong Un, and his intricate knowledge of the various policy issues at play means that U.S. officials will regard what he says as a bellwether for the DPRK’s intentions at the summit.”

The flurry of diplomatic activity was the latest in a series of near-daily developments as the U.S. and South Korea seek to persuade the North to relinquish its nuclear weapons.

Over the weekend, Moon and Kim Jong Un held a secret summit, with the North Korean leader reiterating his commitment to denuclearizing and to a Trump sit-down.

Significant differences, however, remain — especially over the U.S. and North Korean definition of “complete denuclearization.”

Moon has said Kim can be persuaded to agree to Washington’s demand for the complete, verifiable and irreversible dismantlement (CVID) of his nuclear arms and missile programs in exchange for credible security and economic guarantees.

The South Korean leader has admitted that the process of the North giving up its nukes could be difficult even if Pyongyang, Washington and Seoul agree on any definition, but urged Trump and Kim “to settle misunderstandings through direct communication and have sufficient communication through working-level talks on the agenda to be agreed on at the summit,” something he said Kim had agreed with.

While experts say it highly unlikely that the advance team will be able to persuade the North to denuclearize as quickly as Trump wants, negotiating the language and a timetable that both Trump and Kim can agree on remains a distinct possibility.

Pyongyang has said it favors a phased and incremental approach to giving up its nukes, moves that would come in exchange for eased sanctions, formally ending the Korean War and a security guarantee for the regime.