Moon's remarks came after his special security adviser, Moon Chung-in, claimed Seoul and Washington may consider scaling back their joint military drills staged in South Korea should the North freeze its nuclear activities.

The Yonsei University professor later said he was speaking within his personal capacity and not a special adviser to the president.

Still, his remarks sparked concerns that Seoul's new liberal government under the new president may be considering making such an offer to Pyongyang despite the North's continued nuclear and missile provocations.

"I do believe the notion that we must not reward bad behavior is a principle we must uphold," the new South Korean president told reporters.

North Korea has staged five missile tests since Moon came into office on May 10.

Still, Moon noted such a reward may be possible if the North agrees to give up its nuclear ambition and takes verifiable steps towards complete denuclearization.

"The most ideal solution would be to completely denuclearize North Korea in an one-shot deal. But more realistically, I believe such a deal will not be easy," Moon said.

"I believe (the North) must at least promise to a nuclear freeze for us to start taking serious measures (discussions) for its denuclearization. In that sense, its nuclear freeze will be the entrance and nuclear dismantlement the exit," he added.

The South Korean leader said a reduction in his country's joint military exercises with the US could be used as a reward in what he called a step-by-step denuclearization process.

"Each step in the process must be completely verified before we will be able to move onto the next," he said.

Pyongyang had agreed to give up its nuclear programs in the past, once even dismantling the cooling tower of its only known nuclear reactor as part of "irreversible" denuclearization measures in 2008.

The country later returned to nuclear armament, staging two nuclear tests in 2016 alone.

Moon insisted the North's breach of a future denuclearization agreement, should there be one, would give the international community the legitimacy to take "any measure whatsoever."

The South Korean president expressed hope his summit with Trump would mark the start of discussions on ways to rid North Korea of its nuclear weapons.

The two leaders are set to hold their first bilateral talks on Friday.

The summit was also expected to focus on ways to strengthen the Korea-US alliance under the countries' new administration.

Moon noted he and Trump may work together over the next five years should the latter be reelected following his first four-year term that will end in January 2021. Moon's single five-year term ends in May 2022.

Still, topics for the meeting were also expected to include thorny issues, such as a US demand for renegotiation of the countries' bilateral free trade agreement that went into effect in March 2012.

Trump earlier called the Korea-US FTA a "horrible deal."

Moon noted the US president may have been referring to the United States' trade deficit with South Korea, but insisted the deal was greatly benefiting both countries.

Since the Korea-US FTA went into effect, the overall volume of global trade shrank by more than 12 percent, according to Moon.

Bilateral trade between South Korea and the United States, on the other hand, surged 12 percent over the cited period, he noted, apparently implying the free trade pact may have kept both the countries' trade growing.

"FTAs may benefit only certain sectors of certain countries, and we are no exception. The Korea-US FTA benefited our manufacturing, automobile industries, but damaged the agricultural sector," Moon said. (Yonhap)