Kim Yong Chol, a close aide of North Korean leader Kim Jong Un and vice chairman of the ruling Workers' Party's Central Committee, arrived at New York's John F. Kennedy airport on a flight from Beijing. He was later seen entering a hotel in midtown Manhattan.
Kim Yong Chol is due to meet with US Secretary of State Mike Pompeo on Wednesday and Thursday to discuss whether to go ahead with an unprecedented meeting between US President Donald Trump and North Korean leader Kim.
The summit, originally planned for June 12 in Singapore, has been cast in doubt as the two countries - which have technically been at war since the 1950s - test each other's diplomatic mettle.
As the US and North Korean officials prepared for their first encounter in New York on Wednesday evening, Pompeo repeated the Trump administration's demands.
"Looking forward to meeting with Kim Yong Chol in New York to discuss @Potus potential summit with Chairman Kim. We are committed to the complete, verifiable, and irreversible denuclearisation of the Korean Peninsula @StateDept," Pompeo said in a Twitter post.
Trump last week called off the summit after North Korea expressed anger at comments by senior US officials. But Trump later said he was reconsidering his position and US, North Korean and South Korean officials have gone ahead with summit preparations.
The White House said on Wednesday that negotiations at the demilitarized zone along the border between North and South Korea for the summit were going well and that it expects the historic meeting to take place on June 12.
Separately on Wednesday at Pearl Harbor in Hawaii, Admiral Harry Harris, the outgoing head of US Pacific Command who is Trump's pick to be the ambassador to South Korea, said North Korea remained the United States' most imminent threat. "And a nuclear-capable North Korea with missiles that can reach the United States is unacceptable," Harris said.
A US official said negotiations about the summit's agenda have so far been slow going, and the two fundamental issues, the definition of North Korean denuclearisation and whether both sides would take actions at the same time or separately, remained unsettled.
Trump has sworn not to allow North Korea to develop nuclear missiles that could hit the United States and wants North Korea to "denuclearize," or get rid of its nuclear arms.
The US official said the US position remained that Pyongyang must agree to complete denuclearisation before it can receive any financial assistance, such as relaxation of some economic sanctions, new food or other aid, or new investment, and that the degree of assistance would depend on the pace and extent of Pyongyang's actions.
North Korea's position going into the meetings with Pompeo in New York, meanwhile, was that a pledge of denuclearisation alone should open the way to economic relief, said the official, who is familiar with the talks and spoke on condition of anonymity.
South Korea's Yonhap news agency quoted Seoul's Unification Minister Cho Myoung-gyon as saying in a speech on Wednesday that the differences between North Korea and the United States on the denuclearisation issue remained "quite significant" and it would not be easy to narrow the gap.
However, Cho added that the engagement of the leaders of the two countries meant "chances are high that common ground can be found."
Kim Yong Chol is the most senior North Korean official to meet top officials for talks in the United States since Jo Myong Rok, a vice marshal, met then-President Bill Clinton at the White House in 2000.
A former military spy chief, he has played a central role in the thaw in relations over the past six months between North Korea and South Korea, as well as the United States.
During his tenure as a senior intelligence official, Kim was accused by South Korea of masterminding deadly attacks on a South Korean navy ship and an island in 2010. He was linked by US intelligence to a cyber attack on Sony Pictures in 2014.
North Korea denied any involvement in the attack on the ship and on Sony Pictures.
North Korea defends its nuclear and missile programs as a deterrent against what it sees as US ambitions to overthrow its leadership and unite the Korean Peninsula. The United States stations 28,500 troops in South Korea, a legacy of the 1950-1953 Korean War.