A diplomatic source in Washington said the purpose of Suh's visit "was to hold discussions with U.S. intelligence officials."

Haspel will be among U.S. officials to meet North Korean leader Kim Jong-un's right-hand man Kim Yong-chol, who arrived in Washington on Thursday to discuss the agenda and venue of the summit. Officials from the three countries are fine-tuning details of the summit, which is expected in mid-February.

The Korea Mission Center at the CIA and North Korea's United Front Department, which Kim Yong-chol heads, have played leading roles in U.S.-North Korea negotiations. They held several meetings in the border truce village of Panmunjom to fine-tune the agenda, timing and venue of the next U.S.-North Korea summit.

Discussions focused on how the U.S. would respond to denuclearization steps proposed by the North. Diplomatic sources speculate that the U.S. and North Korea may even have reached some sort of agreement through intelligence channels.

Suh's visit to the U.S. was apparently aimed at speeding up these developments. "Suh frequently travels to the U.S.," the source said.

Kim Yong-chol was expected to stay in the U.S. for two days, but it has emerged that he has booked his return flight for Saturday afternoon. He is to meet with U.S. Secretary of State Mike Pompeo and Stephen Biegun, the U.S. Special Representative for North Korea and could also meet President Donald Trump.

Regarding the summit venue, U.S. Representative Michael McCaul told Voice of America that it is likely to be Hanoi. But the Washington Post reported that Da Nang, also in Vietnam, is a more likely location and that the summit will not take place until March or April.