An official from South Korea's presidential office Cheong Wa Dae in Seoul insisted the professor was voicing his own personal views, saying they had not been coordinated with the presidential office, let alone the president.

The Cheong Wa Dae official, speaking to reporters on the condition of anonymity, noted the special adviser had met with a ranking official from the presidential National Security Office prior to his US trip, but said the two had merely exchanged greetings.

Another Cheong Wa Dae official said the presidential office has since contacted the special adviser and sternly warned him of the danger of making such comments even if they were his personal views.

"We sternly spoke of the fact that this may not be helpful to Korea-US relations in the future," the official said, also speaking on condition of anonymity.

"Moon Chung-in's comments will exacerbate US concerns about President Moon Jae-in's potential policies on North Korea and the US alliance," Bruce Klingner, a Korea expert at the Heritage Foundation, told Yonhap News Agency.

"To date, President Moon and most advisers have been reassuring to Washington, their comments suggesting the new administration will not embrace the far-left policies of liberal predecessor Roh Moo-hyun," the expert said.

However, professor Moon's remarks run counter to President Moon's comments, Klingner said, adding that the adviser has described a more unilateral South Korean engagement approach toward Pyongyang and stronger skepticism or even resistance to the THAAD deployment.

"Coming on the eve of the US-South Korean summit, professor Moon's trip could be seen as counter-productive, heightening rather than allaying US concerns. But it also serves to underscore the importance of the Trump-Moon meeting to align allied policy toward the common North Korean threat," he said.

"While it is too early to predict a return to the strained bilateral relations during Roh Moo-hyun's administration, that is certainly a very real potential," he added.

Robert Manning, an Atlantic Council expert, acknowledged that some of the US "show of force" activities, such as flying nuclear bombers over the Demilitarized Zone on the border with the North, have been excessive and counterproductive.

Still, he said it's a "bad idea" to reduce US-Korea military readiness "in exchange for North Korea not doing things it shouldn't be doing and which violate UN Security Council resolutions.

"Some of the ideas floated by President Moon and his top advisers seem to be dusting off old 'sunshine' ideas that failed," Manning said. "While some dialogue with North Korea is needed, so are tougher sanctions."

Manning also said that "freeze" ideas risk being manipulated by the North and stressed that the nuclear issue should be resolved based on the September 2005 Joint Statement that committed Pyongyang to completely ending its nuclear program in exchange for diplomatic and economic concessions.

"If President Moon puts forward some ideas they have floated, it may lead to discord at the Moon-Trump summit," Manning said. (Yonhap)