South Korea's top nuclear envoy struck an upbeat note Thursday on the planned field survey of inter-Korean railways as he returned from a trip to the United States.

Lee Do-hoon, special representative for Korean Peninsula peace and security affairs, also voiced optimism about the possibility of high-level talks between Pyongyang and Washington.

He led the inaugural "working group" talks with Stephen Biegun, the US point man on Pyongyang, in Washington D.C.

The two sides discussed the issue of a joint on-site inspection by South and North Korea of their railways to modernize and re-connect them.

It was supposed to take place in late October but was delayed amid concern about possible negative impact on UN-led sanctions against the communist nation.

"The U.S. expressed very strong support for the survey project," Lee told reporters upon arriving at Incheon International Airport. "Thus, I think there will be good news in the near future."

On the prospect of the North and the US holding high-level talks, Lee said the two sides are in close communication to set a schedule.

"I expect good news (on the matter) as well," he said, without ruling out the possibility of talks before the end of this month.

If President Donald Trump hopes to hold another summit with the North Korean leader Kim Jong-un early next year, there is not much time to prepare for it.

Asked about the role of the working group, Lee said it is meant to prevent either South Korea or the US from "being surprised" by a decision related to North Korea.

The allies plan to hold bilateral "working group" consultations on North Korea as often as possible, either through face-to-face talks or videoconferencing.

They want more systemic and frequent discussions on ways to deal with Pyongyang to coordinate the pace of denuclearization talks and that of inter-Korean cooperation.

The US wants the two things to proceed in tandem, while South Korea says an improvement in its relations with North Korea will help move denuclearization forward.

The working group is intended to focus on four main issues: diplomacy on North Korea, denuclearization, inter-Korean ties and the implementation of UN sanctions.

It involves officials from the presidential offices and foreign ministries of the two sides, as well as South Korea's unification ministry, which deals with inter-Korean cooperation.