Mr. Moon’s office did not reveal what those step-by-step measures were or whether the South would be informed in advance should the United States, for instance, attempt a surgical strike at a North Korean missile site — an action that analysts say could escalate into an armed conflict.
As the war of words escalated, analysts began doubting that Mr. Trump was issuing his provocative statements against North Korea in coordination with allies in Seoul. President Moon also faced accusations from the conservative political opposition and the South Korean news media that Seoul was being ignored by Washington, its most important ally in dealing with North Korea’s nuclear and missile programs.
South Korea’s emphasis on transparency on Friday followed a series of statements opposing an armed conflict with the North. Although most South Koreans loathe the repressive regime in the North and its nuclear pursuit, they also are afraid that the South has more to lose in a war than the impoverished North, a fear that the regime of Kim Jong-un, the North’s leader, has been using as leverage.
After Mr. Moon talked with Mr. Trump on the phone on Monday, his office said he emphasized that “South Korea can never accept a war erupting again on the Korean Peninsula.” On Thursday, his office said the escalation of military tensions or an armed clash would “not help any country.”
Leaders of Mr. Moon’s governing Democratic Party had more pointed jabs for Washington.
“High-ranking American officials too should refrain from using excessive language,” said the party’s chairwoman, Choo Mi-ae, without mentioning Mr. Trump by name. “Their impromptu and not-carefully-thought-out messages only serve to worsen the situation and play into the hands of North Korea’s shrewd intentions.”
Ms. Choo also berated North Korea, calling its plan to target Guam “a delusion that would only bring about its self-destruction.”
She also said that the intensifying threats between the United States and South Korea were hurting investors’ confidence. On Friday, the South’s benchmark Kospi index plummeted 39.76 points, or 1.69 percent, to its lowest level since May 24.
“There is a growing fear that the escalating tension between North Korea and the United States, which matches power with power, could lead to a worst-case scenario,” Ms. Choo said. “All sides in and around the Korean Peninsula must do what they can to turn the situation into a dialogue phase before things get irremediable.”
Some analysts also cautioned against overreacting to Mr. Trump’s tough rhetoric.
“The United States is not going to start a war over South Korea’s head here,” Patrick M. Cronin, senior director of the Asia-Pacific Security Program at the Center for a New American Security in Washington, said on Friday after meeting with White House officials. “The intention is to deter and contain North Korea.”
Mr. Cronin said Mr. Trump’s harsh words were designed to “signal a resolve to demonstrate that deterrence holds and that we have the capability to neutralize Kim should he attack.”
But he said that by using “the same kind of rhetoric that North Korea is long accustomed to using,” Mr. Trump also ends up unintentionally helping Mr. Kim’s strategy of creating the fear “that he could instigate war.”