"Before the process of denuclearization can be final, we must have a complete understanding of the full extent of the North Korean WMD and missile programs through a comprehensive declaration," Biegun said during a speech at Stanford University, referring to weapons of mass destruction.

He said the US has "contingencies" prepared in case negotiations fail, but did not give details.

North Korea has balked at such a demand, claiming an inventory will give the US a list of targets to destroy.

Biegun made clear the US also intends to verify denuclearization.

"We must reach agreement on expert access and monitoring mechanisms of key sites to international standards, and ultimately ensure the removal or destruction of stockpiles of fissile material, weapons, missiles, launchers and other weapons of mass destruction," the envoy said.

The US and North Korea reached a vague denuclearization deal during last June's first summit between US President Donald Trump and North Korean leader Kim Jong-un.

The agreement committed the North to work toward complete denuclearization of the Korean Peninsula in exchange for security guarantees for Pyongyang, without laying out concrete steps.

In late February, the two leaders are set to meet again, reportedly in Vietnam, to try to flesh out the deal. Trump said earlier Thursday that the exact date and venue will be announced early next week.

Many North Korea watchers in the US have warned that the two sides will need to agree on a detailed implementation plan before the second summit or risk giving the regime the legitimacy it seeks as a nuclear weapons state.

Biegun said US Secretary of State Mike Pompeo was promised by Kim during his October trip to Pyongyang that the regime would dismantle and destroy its plutonium and uranium enrichment facilities.

He also said that the US has told the North Koreans that it is prepared to carry out the commitments from Singapore's first summit "simultaneously and in parallel."

The offer is in line with North Korea's wishes as the regime has accused the US of making "gangster-like" demands to first surrender all of its nuclear weapons for only a promise of future economic concessions.

Biegun noted that the US has eased rules for the delivery of humanitarian aid to North Korea, which was previously delayed by United Nations sanctions on the regime.

But he reiterated that the US will keep punishing sanctions on the North until it sees complete denuclearization.

Aside from denuclearization, Trump and Kim also committed at their first summit to pursue "new" relations between the countries, establish peace on the peninsula and work for the return of the remains of American soldiers killed in the 1950-53 Korean War.

Biegun is set to travel to Seoul on Sunday for working-level meetings with his North Korean counterpart, Kim Hyok-chol, ahead of the second summit. The State Department, in announcing the trip, did not reveal when or where the talks will be held, but they are expected to take place in the inter-Korean border village of Panmunjom.

The two men were introduced earlier this month when Kim Hyok-chol accompanied a senior North Korean official to Washington to meet with Trump and Pompeo.

That first meeting with Kim, previously Pyongyang's ambassador to Spain, was "productive, focused, results-oriented, and it laid out first steps in a plan for comprehensive working-level negotiations ahead," Biegun said.

"We were satisfied with the outcome of the visit and in the very near future we'll be pursuing concrete plans to advance all of the elements of the Singapore joint statement," he added.

In particular, next week's meeting will discuss "corresponding measures" the US can offer in exchange for denuclearization.

Biegun went further to assure the North that the US has no interest in regime change. "We're not going to invade North Korea," he said.

The two nations fought each other in the Korean War, leaving a legacy of American troops stationed in South Korea to deter North Korean aggression.

Pundits have expressed concern that Trump, who has complained about the cost of keeping 28,500 troops in the ally nation, could agree to pull them out as part of a new deal with the North.

According to Biegun, the topic has not been discussed with the regime.

He also acknowledged that there remains a difference in the two sides' interpretation of "denuclearization."

North Korea has traditionally viewed it as a term meaning not only the elimination of its nuclear arsenal but also the end of the US nuclear umbrella over South Korea. (Yonhap)