Officials were surprised Tuesday when President Donald Trump declared that the U.S. would respond with “fire and fury” to continued threats by North Korea hours after a report said the country had the ability to miniaturize nuclear weapons, according to multiple reports.

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The comment, which heightened tension between the U.S. and North Korea and prompted Pyongyang to threaten action on Guam, was not discussed specifically with his advisers beforehand, The New York Times reported, citing unidentified sources with direct knowledge of the situation.

“He had talked over possible responses in a general way,” the newspaper reported.

The Washington Post reported that U.S. officials determined last month that North Korea has produced a nuclear warhead small enough to fit inside its missiles.

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“The (intelligence community) assesses North Korea has produced nuclear weapons for ballistic missile delivery, to include delivery by ICBM-class missiles,” an excerpt of an unrelated analysis from the Defense Intelligence Agency said, according to the Post.

Hours after the report was posted online, Trump told journalists in New Jersey that any threats against the United States would be met with “fire and fury like the world has never seen.”

"President Trump's comment was unplanned and spontaneous," a senior administration official, who spoke on the condition of anonymity, told Reuters.

Another unnamed official told the news wire that the comment was “all Trump.”

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Secretary of State Rex Tillerson appeared to try to minimize the president’s comments on Wednesday, telling reporters that North Korea does not present an imminent threat.

"Americans should sleep well at night," Tillerson said. "Nothing that I have seen, and nothing that I know of, would indicate that the situation has dramatically changed in the last 24 hours."

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In a separate, forceful statement released Wednesday, Secretary of Defense James Mattis said that North Korea “should cease any consideration of actions that would lead to the end of its regime and the destruction of its people."

"While our State Department is making every effort to resolve this global threat through diplomatic means, it must be noted that the combined allied militaries now possess the most precise, rehearsed and robust defensive and offensive capabilities on Earth,” Mattis said.