A senior North Korean official was due to visit the White House Friday to deliver a letter from his leader Kim Jong-un to U.S. President Donald Trump.

Kim Yong-chol, known as a close aide to Kim Jong-un, was headed to Washington from New York, where he held two days of meetings with U.S. Secretary of State Mike Pompeo on a possible summit between their leaders.

Pompeo said after the meetings that "real progress" was made toward "setting the conditions" for a summit, but he also said there remains "a great deal of work to do."

Depending on the what the letter says, Trump could decide whether to go ahead with the June 12 summit in Singapore as originally planned.

The two sides are at odds over how far North Korea will go in dismantling its nuclear weapons program and what security guarantees the regime would receive in return.

The U.S. wants to see the North quickly and "irreversibly" dismantle the program before providing significant concessions, but the North has rejected "unilateral" abandonment.

Trump called off the summit in an open letter to Kim last week, citing "open hostility" from the regime. He has since put it back on track amid a flurry of diplomacy to salvage the potentially historic meeting.

Trump announced Thursday that he was expecting the North Korean delegation to deliver a letter from Kim. He said he looks forward to seeing "what's in the letter" and thinks it will be "very positive."

Kim Yong-chol, vice chairman of the central committee of North Korea's ruling Workers' Party, is the highest-ranking North Korean to visit the U.S. since 2000.

That year Vice Marshal Jo Myong-rok visited the White House and met with then President Bill Clinton. (Yonhap)