Chung Eui-yong, head of Seoul's National Security Office, will meet top US officials including National Security Advisor HR McMaster and Secretary of State Rex Tillerson, Yonhap news agency said.

"We have a separate message from North Korea for the US," Chung told journalists after revealing details of his meeting with the North's leader Kim Jong Un.

In a surprising turnaround, Kim said -- as relayed by Seoul -- that the North wanted to talk to the US and would not need nuclear weapons if the country's security was guaranteed.

The two Koreas also agreed to hold a third inter-Korean summit in late April at the southern side of the border truce village of Panmunjom.

Chung was accompanied Thursday by Suh Hoon, the chief of South Korea's spy agency, the National Intelligence Service, who was also part of the delegation to Pyongyang.

After Washington, Chung will later visit China and Russia to brief officials, while Suh will travel to Japan.

The three countries, along with the US, are involved in six-party talks on ending North Korea's nuclear drive which have stalled since 2008.

US President Donald Trump welcomed this week's developments as "very positive" and said the North's talks offer appeared to be "sincere", adding: "We'll soon find out."

The Dong-A Ilbo newspaper quoted an unidentified senior official of the presidential Blue House saying that Kim had not made any specific "give-and-take style" demands concerning dialogue with the US.

"He is apparently drawing a big picture," the official was quoted as saying.

"Kim has expressed his willingness to make his country a normal state" instead of a pariah state under sanctions, he added.

Analyst Cheong Seong-chang of the Sejong Institute said the landmark visit to Pyongyang had provided "key momentum" to contain the North's nuclear and missile threats, prevent conflict on the Korean peninsula and and start building trust.

"However, the path toward denuclearisation will be very rough," he said, forecasting that the North would continue missile production this year even if it stops nuclear and rocket tests.

And he added: "Even if an agreement is reached on freezing the North's nuclear programmes, complete verification would be next to impossible."