The acting US ambassador to South Korea said Monday a complete, verifiable and irreversible dismantlement of North Korea's nuclear program remains a "nonnegotiable" US goal in its upcoming talks with Pyongyang.

During a forum in Seoul, Marc Knapper said the US is "hopeful but realistic" in its expectations for the upcoming summit between the leaders of the US and North Korea, citing more than two decades of failure in making the North give up its nuclear weapons.

"These are not sanctions for sanctions. The goal of this pressure campaign is and always has been to persuade North Korea to engage in meaningful dialogue about a different future for itself and its people," Knapper said. "To that end, we sincerely welcome recent developments that are potentially leading to a resolution of the nuclear issue.

"But let me be clear. There has been no shift in our policy. We are willing to engage with North Korea but our purpose of any meeting first and foremost will be to emphasize the complete, verifiable and irreversible denuclearization of the Korean Peninsula. It is necessary and nonnegotiable," he added.

South Korea and the US prepare for their respective summits with North Korea in April and May to discuss the nuclear issue with Pyongyang. South and North Korea announced earlier that their leaders will meet at the truce village of Panmunjom on April 27.

Speculation has arisen that South Korea and the US might not be on the same page in approaching the denuclearization of North Korea, as Seoul officials reportedly prefer a "step-by-step" approach in ridding the North of its nuclear weapons but Washington apparently pursues the so-called Libya-style method where the North should give up its nuclear program before discussing any possible concessions.

Knapper dismissed the concerns, saying that the US and South Korea are "fully aligned."

"The US and the ROK (South Korea) are fully aligned when it comes to our approach to North Korea. We believe very strongly and agree strongly with President Moon's approach, which is that there will be no progress in North-South relations without progress on denuclearization."

He cautioned against reading too much into such speculation and advised tempering expectations about the back-to-back summits with the reclusive state.

"We sincerely hope that these summits ... will lead to the kind of progress that we failed to achieve for the past 20-25 years. So yes, we are hopeful but we are also realistic," he said.

"Many of us for the past 20-25 years who have dealt with the issues have seen the promise of progress and its failure because of North Korea's failure to abide by the terms of the agreements. So we are hopeful but we have to be realistic and we have to be open-eyed as we go forward," he added.

He said that it is meaningful that North Korean leader Kim Jong-un has apparently expressed his commitment to denuclearization for the first time but noted that the US still needs to "know more" about what he actually has in mind.

"This is the first time that we have heard this reference to commitment ... Apparently he said the same ... Kim Jong-un said the same thing to (Chinese President) Xi." he said. "We wanna know more. We need to know more ... The bottom line has been, is and will remain until we sit down ... is CVID. We can't accept anything less." (Yonhap)