North Korea's top envoy involved in denuclearisation talks with the United States boarded a flight to Washington from Beijing on Thursday, South Korea's Yonhap news agency said.
US and South Korean media previously quoted sources as saying US Secretary of State Mike Pompeo and North Korean official Kim Yong Chol were expected to meet in the US capital on Friday to discuss a second summit between President Donald Trump and North Korean leader Kim Jong-un.
Mr Pompeo had planned to meet his North Korean counterpart to discuss a second summit last November, but the meeting was postponed.
Kim Yong Chol was last in Washington in June, when he delivered a letter from Kim Jong-un to Mr Trump that opened the way for an unprecedented meeting between the leaders of the two countries in Singapore on June 12.
CNN quoted a source familiar with the US-North Korea talks as saying that Kim Yong Chol would be carrying a new letter from Kim Jong-un to Mr Trump.
Chinese and South Korean envoys on Korean peninsula affairs met in Seoul on Thursday, the South's foreign ministry said. Kong Xuanyou and Lee Do-Hoon were expected to have discussed ways to achieve denuclearisation.
In Singapore last year, Kim Jong-un pledged to work towards denuclearisation, but there has been little significant progress since.
Contact was resumed after the North Korean leader delivered a New Year speech in which he said he was willing to meet Mr Trump "at any time", South Korea's ambassador to the US, Cho Yoon-je, told reporters last week.
Communist-ruled Vietnam, which has good relations with both the United States and North Korea, has been widely touted as the most likely venue for a second meeting.
The South-East Asian country is keen to host the summit as a demonstration of its normalised ties with the United States, its foe during the Vietnam War, a Vietnamese government official and two diplomatic sources told Reuters.
Vietnam is already preparing for a state visit by Kim Jong-un that is expected to take place after the February 4-8 Lunar New year celebration, the sources said.
The Vietnamese source did not confirm if the state visit would be combined with a US-North Korean summit.
Media is heavily censored in Vietnam and the ruling Communist Party retains a tight control on information. The officials who spoke to Reuters did so on the strict condition of anonymity.
Vietnam's foreign ministry did not respond to a request for comment.
A US embassy spokeswoman in Hanoi said the embassy did not have anything to announce regarding the summit location, and referred questions to the White House.
Mr Trump said on January 6 that the US and North Korea were "negotiating a location" for a second summit, and that "it will be announced probably in the not too distant future". White House officials have declined to comment further.
Both Hanoi and the central Vietnam city of Danang have been viewed by analysts as possible locations for a second summit.
A senior Danang-based official said the city had not been asked to host such an event, but had received instructions to prepare for a possible "A1" visit, referring to a high-profile foreign leader.
A senior South Korean official earlier told Reuters that the United States has "various options" on the location, which would be decided after consultations with North Korea, while another official said that Vietnam is only one of the countries keen to host the summit.