South Korea's top nuclear envoy left Tuesday for the United States to discuss ways to maintain momentum in efforts to denuclearize North Korea following a no-deal Hanoi summit.

Lee Do-hoon, special representative for Korean Peninsula peace and security affairs, said he plans to meet with his American counterpart, Stephen Biegun, and other officials in Washington, DC.

"In the process, (we) plan to analyze the outcome of the North Korea-US summit in Hanoi, solidifying coordination between South Korea and the US, and consult on what to do going forward," he told reporters at Incheon International Airport, west of Seoul, shortly before departure.

Concern has grown that the denuclearization talks will lose steam, as the second summit between North Korean leader Kim Jong-un and US President Donald Trump concluded without any agreement.

South Korean President Moon Jae-in stated that his administration will try to play an active mediating role again between Pyongyang and Washington.

A government-civilian forum, known as Track II diplomacy, among the Koreas and the US is cited as a realistic option to help kick-start denuclearization talks.

The three nations' senior officials had a gathering in Sweden in January on the sidelines of an international security forum joined by civilian experts.

Lee, however, said it's too early to speak publicly about any related specific plan.

He was also guarded about whether he believes inter-Korean economic cooperation will be able to serve as a catalyst for Pyongyang-Washington discussions.

Lee's ministry later emphasized that Seoul hopes for an early resumption of Pyongyang-Washington dialogue.

The nuclear envoy's trip to the US is in line with South Korea's "expectations and efforts," the foreign ministry's spokesman Noh Kyu-duk said at a regular press briefing.

He added there are various ideas being discussed and the Track II diplomacy is believed to be one of the "effective" ways.

"Still, we are not at a stage that any specific plan has been already made," he said.

Lee's visit this time is not for a "working group" session between the two sides, their diplomatic consultation channel focusing on the North Korea issue, according to the spokesman, who said they will soon schedule a relevant formal meeting.

Lee is scheduled to stay in the US capital through Thursday. (Yonhap)