US Deputy Secretary of State Stephen Biegun said Thursday that diplomacy remains the "best" and "only" course to address the North Korean nuclear quandary, calling for Pyongyang to return to dialogue.

Biegun, who doubles as Washington's top nuclear envoy, made the remarks during a lecture in Seoul recapping his yearslong, unfinished negotiations with the North, which he cast as being marked by "setbacks, disappointments and missed opportunities."

"As we look to the future, I remain convinced that diplomacy remains the best course, indeed the only course, to solving our challenges with North Korea," he said during the lecture at the local think tank Asan Institute for Policy Studies.

"Pyongyang has some pivotal events coming up, in particular the eighth party congress in January. We strongly encourage North Korea to use the time between now and then to set a path for the resumption of diplomacy," he added.

The US envoy was referring to next month's key session of the North's ruling Workers' Party, at which Pyongyang could unveil a new national strategy to ride out a series of challenges, including the COVID-19 pandemic, global sanctions pressure and economic travails.

Asked about Washington's rejection of Pyongyang's offer to dismantle its mainstay Yongbyon nuclear complex during their summit in Hanoi early last year, Biegun stressed that it does not represent the entirety of the North Korean nuclear program.

"Of course, Yongbyon remains a great interest to the United States of America, and that's a historic center of the North Korean nuclear weapons program. It still retains great importance in any process of the complete denuclearization of the Korean Peninsula," he said.

"But the challenge that we faced in Hanoi, the challenge we face to this day, is it is not all of North Korea's nuclear weapons program," he added.

The deputy secretary, however, pointed out that the US has "never expected one side has to do everything before the other side does anything."

As for the "key" lesson from the no-deal summit in Hanoi, Biegun underscored that his North Korean counterparts were "not empowered" to discuss serious negotiation topics.

"We didn't have enough time in Hanoi to fully explore the limits of what might be possible, in part because the negotiating team that arrived before the summit was not empowered to discuss those issues," he said.

The US envoy also expressed regrets over North Korean counterparts missing opportunities just in "search for obstacles."

"Regrettably, much opportunity has been squandered by our North Korean counterparts over the past two years, who too often have devoted themselves to the search for obstacles to negotiations instead of seizing opportunities for engagement," he said.

"Yet, remarkably, the potential of the Singapore Summit is still fully present, despite our failure to advance what was agreed."

At the landmark 2018 summit between US President Donald Trump and North Korean leader Kim Jong-un in Singapore, they agreed to work toward the "complete" denuclearization of the peninsula, seek to build new bilateral ties and make joint efforts to build a lasting and stable peace regime on the peninsula.

But their nuclear talks have been stalled since last year's Hanoi summit. Inter-Korean cooperation has also come to a standstill as Washington keeps tough sanctions in place against the North.

Biegun reiterated that Washington welcomes inter-Korean cooperation to help catalyze lackluster peace efforts.

"We also continue to welcome mutually beneficial inter-Korean cooperation as an important tool to cultivate trust and build cooperation on the Korean Peninsula, and people-to-people exchanges in the fields of education, music, culture and sports would be beneficial to all," he said.

Touching on the bilateral alliance, Biegun stressed that the two countries are "democratic anchors in the Indo-Pacific region."

"The US-ROK Alliance could advance a Pax Indo-Pacifica, a region at peace, protected and made prosperous in equal measure by those who comprise the Indo-Pacific," he said, referring to South Korea's official name, the Republic of Korea.

"This is our purpose, this is our interest, and with strong and visionary leadership, this will be our destiny," he added.

Wrapping up his speech, he enumerated some points he will deliver to his successor in the incoming Joe Biden administration.

"The war is over. The time for conflict has ended, and the time for peace has arrived. If we are to succeed, we must work together -- the United States, the Republic of Korea and the DPRK," he said, referring to the North's official name, the Democratic People's Republic of Korea.

"And when we do, we will at long last be able to bring to this peninsula the lasting peace and prosperity that all the Korean people so richly deserve," he added.

Earlier in the day, Biegun stressed Seoul's role and the importance of inter-Korean relations in building peace on the Korean Peninsula during his talks with Unification Minister Lee In-young and said the door to dialogue with North Korea remains open, officials here said.

Biegun arrived here on Tuesday for what could be his last visit in his current capacity as the State Department's No. 2 official and its point man on the North.

He will depart for the United States on Saturday morning. (Yonhap)