"He opened the door back up or at least clarified his position that he was interested in negotiations," Lippert told a defense-related forum here. "In the last three months, my prognostication of military action was higher because it seemed as though negotiations were off the table and the national security adviser was saying in public that deterrence and other forms of containment aren't viable options."

Tensions spiked after North Korea test-fired two long-range missiles in July and detonated its sixth and most powerful nuclear device in September. Trump engaged in a war of words with the North Korean leadership, saying the US may have no choice but to "totally destroy" the communist nation if it continued to pursue a nuclear-tipped missile capable of reaching the American mainland.

"That quickly leads you into a military type of space," Lippert said. "I think after the speech -- lower -- but for the last couple of months, I was a little higher."

The former ambassador stressed the importance of keeping close coordination between Seoul and Washington.

"If you don't get the US-ROK (Republic of Korea) alliance relationship right, it's virtually impossible to run an effective North Korea strategy these days," he said.

Trump's two-day visit to Seoul this week appeared to have relieved some concerns in South Korea that it was being bypassed in efforts to resolve the North Korean problem.

"So I think (it was) a very good, very strong trip, but the word in Korea -- 'sukjae' -- there's some homework left to be done," he added, calling for a clear vision of the alliance going forward in terms of the bilateral free trade agreement and the transfer of wartime operational control of South Korean troops from Washington to Seoul. (Yonhap)