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American muscle-flexing, such as tweeting photos of a B-2 bomber flying over South Korea — which the U.S. Embassy in Seoul did last week — was unlikely to dissuade North Korea from heightening its already antagonistic posture, said John McCreary, a retired military intelligence analyst and longtime Korea watcher.

“The announced moves are not deterrent to the North Koreans, they are provocative and escalatory,” said Mr. McCreary, now with Kforce Government Solutions, a private firm based in Virginia.

Others cautioned against registering the recent threats made by Pyongyang as indicative of a shift toward a new and offensive posture under Kim Jong-un.

North Korea’s latest bellicosity is unusually violent, but nothing suggests that this Kim, any more than his father or grandfather, is suicidal,” Doug Bandow, who specializes in foreign policy at the libertarian Cato Institute, wrote in an analysis published by U.S. News and World Report on Wednesday.

While he said war still could break out via mistake or miscalculation, Mr. Bandow argued that “Washington gains nothing from fixating on the intentions of a bankrupt and backward state which has little ability to strike Americans, except those Washington has voluntarily placed within range — the 28,500 military personnel stationed in South Korea.”

But this week’s uptick in tension also included a report by North Korea’s state-run news agency that the nation’s atomic energy department has plans to “readjust and restart” all of the nuclear facilities at its main nuclear complex, in an area north of Pyongyang known as Yongbyon.

Secretary of State John F. Kerry responded to that report Tuesday by warning that a reactivation of the facility would be “in direct violation” of North Korea’s international obligations and a “provocative act.”

A report by the “38 North” publication produced by Johns Hopkins University’s School of Advanced International Studies sent questions swirling through some corners of Washington’s foreign policy community Thursday on the extent to which North Korea already may be preparing operations at Yongbyon.

“Commercial satellite imagery” indicates that North Korea actually has begun “new construction at a plutonium production reactor located at the Yongbyon nuclear complex that may be intended to restart the facility,” the report says.

Patrick Cronin, who heads the Asia-Pacific Security Program at the Center for New American Security think tank in Washington, called the report “not at all surprising” and said “38 North” was as “reliable an open source of information as you’re going to get.”

The deeper question centers on how important the development actually is, said Mr. Cronin, who added that “It’s what [the North Koreans] are not showing us that I’m more worried about.”

The State Department and Pentagon, meanwhile, have suggested that a core part of the administrations Korea strategy is to gently push China to play a more active role in steering Pyongyang away from provocations and threats that may ultimately provoke military conflict.

Ms. Nuland said Thursday that North Korea “has been the subject of intense conversations” between Mr. Kerry and his Chinese counterparts ahead of the secretary of state’s visit to Beijing later this month.

Susan Crabtree continued to this report.