During a meeting with party leaders, Moon also said he has no plan to ease sanctions against Pyongyang for the sake of his summit with North Korean leader Kim Jong-un slated for late next month.
Moon hosted the lunch meeting with the leaders of five political parties, including the ruling Democratic Party and the main opposition Liberty Korea Party, to explain the results of his special envoys' two-day visit to the North this week.
"Our goal is denuclearization. We cannot settle for nonproliferation or a nuclear freeze," Moon was quoted by party officials as saying.
"The dismantlement of the (North's) nuclear program is the end goal. But given that the immediate dismantlement of it may be difficult, I think we can go through a certain road map before reaching that dismantlement stage."
Moon is seen as pursuing a phased strategy to denuclearize the North that involves the North's initial moratorium on nuclear and missile tests before the dismantlement of its nuclear program. His critics argue that Seoul could settle for a nuclear freeze or end up allowing Pyongyang to be recognized as a nuclear power.
Asked if Seoul proposed any phased approach during his envoys' visit, Moon said no.
President Moon Jae-in (3rd from R) hosts a meeting with the leaders of five major parties at the presidential office Cheong Wa Dae in Seoul on March 7, 2018. (Yonhap)
During the envoys' visit, the two Koreas agreed to hold the summit in late April and set up a hotline between their leaders. The North has also shown its willingness to talk with the United States about its denuclearization and pledged not to use military weapons, either nuclear or conventional, against the South.
Commenting on what will be the third inter-Korean summit, Moon said he may not be able to find much common ground with the North's reclusive leader.
"The summit will take place against the backdrop of international sanctions and pressure," he said. "The room (for cross-border agreements) at the summit will become broader when there is progress in dialogue between the U.S. and the North."
Aware of the worries that cross-border rapprochement could weaken unilateral and international sanctions against the North, Moon said that Seoul cannot unilaterally lift them.
"The ongoing sanctions and pressure against the North are U.N. Security Council sanctions and unilateral sanctions imposed by the U.S.," he said. "The South Korean government is not in a position to unilaterally unravel the standalone sanctions among others."
Moon also rejected speculation that there were secret agreements his envoys reached with the North during their trip to Pyongyang.
"There were never secret agreements," he said.
He stressed the importance of cooperation with Washington in the efforts to foster peace.
"I believe denuclearization talks will materialize when the South and the U.S. are on the same page," he said.
