"At this moment, to achieve further progress, the most important step we can take is for the United States and North Korea to work together to overcome the policies and demonstrations of hostility that compromise the simple ability of our diplomats to talk, and to sustain the rhythm of negotiations," Biegun said in the speech, which was broadcast online.

"If we are to succeed, North Korea must set aside its search for obstacles to negotiations and instead seek opportunities for engagement while that opportunity lasts. We have made clear to North Korea: we are prepared to engage as soon as we hear from them. We are ready. But we cannot do this by ourselves," he said.

If North Korea clings to its weapons of mass destruction, the US and the rest of the world "will not accept that," Biegun warned, urging the North to return to talks to discuss specific actions each side can take to implement the agreements reached at the first Trump-Kim summit in Singapore in June 2018.

Aside from committing the North to work toward complete denuclearization of the Korean Peninsula, the Singapore summit also produced agreements to establish new relations between the countries, build lasting peace on the peninsula and recover and repatriate the remains of American soldiers killed in the 1950-53 Korean War.

As the diplomatic process got under way last year, North Korea also suspended its tests of nuclear weapons and intercontinental ballistic missiles, but recent weeks have seen a renewal of tensions over its testing of short-range ballistic missiles.

Biegun traveled to South Korea last month as US-South Korea military exercises were coming to an end, the point when Kim had stated in a letter to Trump that he would be willing to resume talks with the US

There was speculation the US envoy would use his trip to South Korea to also meet with North Korean officials at the border, but that apparently didn't happen.

Biegun underscored the threat a nuclear-armed North Korea would pose to the region by forcing other countries, such as South Korea and Japan, to consider nuclear armament themselves.

"At what point will voices in South Korea or Japan and elsewhere in Asia begin to ask if they need to be considering their own nuclear capabilities? And what will this mean for a region whose prosperity and growth has been so inextricably tied to long-term stability and peace?" he asked in his speech.

If tensions can be lowered, US military forces in South Korea will no longer need to stand perpetually ready to fight, he added.

"They could instead serve and cooperate to build a foundation to support a lasting peace. And if we can forge sustainable peace, forge the modalities of cooperation, we will reap the mutual rewards that will spring from frank discussion," Biegun said. "This is President Trump's vision, and it is a vision he is confident Chairman Kim shares." (Yonhap)