President Trump on Tuesday sternly warned North Korea that it would face “fire and fury like the world has never seen” if it continues to threaten the United States, as his administration grapples with unsettling news of a possible nuclear breakthrough by the Stalinist regime.

“North Korea [had] best not make any more threats to the United States,” Trump told reporters at his golf resort in Bedminster, N.J. “They will be met with fire and fury like the world has never seen.”

North Korean leader Kim Jong Un “has been very threatening, beyond a normal statement,” Trump said, “and as I said, they will be met with fire, fury and, frankly, power the likes of which this world has never seen before.”

The president’s comments came after multiple U.S. news reports and one Japanese government report suggested that North Korea may have developed a miniaturized nuclear warhead that can fit atop one of its intercontinental ballistic missiles. The U.S. reports, citing anonymous American officials, hinged on an undisclosed Defense Intelligence Agency analysis from last month. The Washington Post was the first to report on the development.

It’s unclear whether North Korea has successfully tested such a miniaturized warhead, though it has boasted of doing so. And it also remains unclear whether the regime in Pyongyang has developed a reentry vehicle that would prevent its warheads from burning up upon entering the atmosphere. The DIA is just one of 17 intelligence agencies, and the reports did not spell out how confident officials are of their conclusion or whether it’s broadly shared across the U.S. government.

Over the weekend, the Trump administration notched a diplomatic victory when the 15-member U.N. Security Council unanimously voted to impose a tough new regime of economic sanctions on North Korea. That vote came in response to the test of an ICBM thought to be able to reach U.S. soil.

In a weekend interview with MSNBC, Trump national security adviser Gen. H.R. McMaster warned that letting North Korea develop nuclear weapons that can strike the United States “is intolerable from the president’s perspective.” McMaster said he was providing Trump with “all options” to respond to the situation, “and that includes a military option.”

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