President Donald Trump said North Korea will be “met with fire and fury and, frankly, power the likes of which the world has never seen before” if Kim Jong Un’s regime continues to threaten the U.S.

“He has been very threatening,” Trump told reporters in New Jersey on Tuesday.

Trump’s comments followed a report in the Washington Post, citing a Defense Intelligence Agency analysis, that Pyongyang successfully developed a miniaturized nuclear warhead that could fit onto its missiles. And it comes just days after the UN Security Council ratcheted up sanctions on North Korea, targeting about $1 billion of the nation’s approximately $3 billion in exports. Those restrictions followed two intercontinental ballistic missile tests in July.

The two developments — test flights of an ICBM and progress on the warhead — add to the pressure on Trump to respond to accelerating North Korea provocations. During the campaign, Trump weighed in on the threat of Pyongyang developing a nuclear warhead capable of reaching the U.S.: “It won’t happen,” he vowed on Twitter.

While other technological hurdles remain for North Korea, Kim’s weapons program appears to be making progress faster than U.S. intelligence agencies previously forecast.

“Today North Korea became only the third U.S. adversary, after Russia and China, to attain the ability to threaten nuclear war against the United States,” said Matthew Kroenig, a nonresident senior fellow in the Brent Scowcroft Center on International Security at the Atlantic Council

Trump’s comment came as Secretary of State Rex Tillerson continues visiting allies in Southeast Asia, rallying support for tougher measures against Pyongyang and holding out an olive branch to the reclusive regime as well. Tillerson was in Thailand Tuesday and heads to Malaysia before returning to the U.S.

“And we hope, again, that this ultimately will result in North Korea coming to the conclusion to choose a different pathway, and when the conditions are right that we can sit and have a dialogue around the future of North Korea so that they feel secure and prosper economically,” Tillerson told reporters in Manila on Aug. 7.