"The phrase that I would seek clarification on is whether North Korea would agree to what they agreed to in September of 2005 -- which was not just denuclearization of the Korean Peninsula -- it is the commitment in writing that North Korea would abandon all nuclear weapons and existing nuclear programs," Cha said. "That is a different phrasing from this very broad 'denuclearization of the Korean Peninsula.'"

US President Donald Trump accepted North Korean leader Kim Jong-un's invitation to meet after the latter conveyed a commitment to denuclearization through a group of South Korean envoys.

Trump said earlier Monday that the historic meeting will take place in May or early June.

Cha pointed to the difficulties of verifying any agreement the two sides may reach on the North's nuclear program.

"In the last agreement, the six-party agreement, this is where it all eventually broke down because we were at the point where they provided a declaration, and that declaration was clearly not a declaration of all of their capabilities that would then have to be verified," he said, referring to the 2005 deal that involved both Koreas, the US, China, Japan and Russia.

"This is clearly going to be one of the biggest obstacles, if we even get that far, in some sort of negotiation with them on the weapons and on the materials," he added.

On what may have prompted Kim to propose the summit, Cha cited a combination of factors, including the effect of growing international sanctions on the regime and the Trump administration's talk of possible military action.

"I also think they probably calculated they could take a pause in their testing; that they have already announced they're a nuclear weapons state; they've already announced they feel that they can threaten the entire US with nuclear ballistic missiles; and they probably felt they have reached a point where there are other things they may need to do, but they don't need to do it right now," he said.

Moreover, the North Koreans have been eager to meet the US president as a nuclear weapons state.

"I think the North Koreans have been waiting for this meeting for four decades," he said. "Even if the discussion on the table is about denuclearization, I think they have been seeking this meeting, seeking the status of this meeting for a long time. Just the meeting in and of itself, I think, the North Koreans will consider a victory."

Speaking at the same event, retired Adm. Mike Mullen, former chairman of the Joint Chiefs of Staff, addressed whether the US

should consider withdrawing its 28,500 troops from South Korea in exchange for North Korea's denuclearization.

"Long term, obviously, if we're able to sort through the security issues in our region and in particular with our allies -- both South Korea and Japan -- and if we are at a point where the possibility of conflict, if you will, has diminished dramatically, I'd certainly be willing to have a discussion about how long the US troops should stay there, and how many of them should be there," he said.

Mullen noted it would not happen anytime soon.

"It is an unbelievably strong strategic commitment on the part of the United States of America to the security of the South Korean people, so it is not anything that could happen quickly," he said. (Yonhap)