U.S. Secretary of State John Kerry and his South Korean counterpart say North Korea will gain nothing by threatening tests of its missile or nuclear programs.

Mr. Kerry says the U.S. and its Asian ally won’t accept the North as a nuclear power. And he says its rhetoric is “unacceptable.”

Mr. Kerry is making his first-ever visit to Seoul amid strong suspicion that North Korea may soon test a mid-range missile.

South Korean Foreign Minister Yun Byung-se called Pyongyang’s threats a “grave provocation” to the entire international community.

Earlier, Mr. Kerry arrived in South Korea on Friday on an unusual diplomatic journey, travelling directly into a region bracing for a possible North Korean missile test and risking that his presence alone could spur Pyongyang into another headline-seeking provocation.

Mr. Kerry was kicking off four days of talks in East Asia amid speculation that the North’s unpredictable regime would launch a mid-range missile designed to reach as far as the U.S. territory of Guam. Mr. Kerry also planned to visit China and Japan.

Mr. Kerry’s trip coincides with the disclosure of a new U.S. intelligence report that concludes North Korea has advanced its nuclear knowhow to the point that it could arm a ballistic missile with a nuclear warhead. The analysis, disclosed on Thursday at a congressional hearing in Washington, said the Pentagon’s intelligence wing has “moderate confidence” that North Korea has nuclear weapons capable of delivery by ballistic missiles but that the weapon would be unreliable.

Pentagon spokesman George Little said afterward that “it would be inaccurate to suggest that the North Korean regime has fully tested, developed or demonstrated the kinds of nuclear capabilities referenced” at the congressional hearing.

James Clapper, the director of national intelligence, said he concurred with Little and noted that the report alluded to at the hearing was compiled by the Pentagon’s Defense Intelligence Agency and was not an assessment by the entire U.S. intelligence community. “Moreover, North Korea has not yet demonstrated the full range of capabilities necessary for a nuclear armed missile,” he said.

President Barack Obama on Thursday urged calm, calling on Pyongyang to end its sabre-rattling while sternly warning that he would “take all necessary steps” to protect American citizens.

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