North Korean leader Kim Jong Un made a rare, explosive address Thursday, two days after U.S. President Donald Trump threatened to “totally destroy” the isolated nation if provoked.

“I will surely and definitely tame the mentally deranged U.S. dotard with fire,” Kim said of Trump, who is 71 years old. “Action is the best option in treating the dotard who, hard of hearing, is uttering only what he wanted to say,” he continued, according to a translation of his statement.

Just hours later, that call for action was reiterated by the country’s foreign minister, who threatened the test of a hydrogen bomb somewhere in the Pacific, South Korean media reported.

Kim Jong Un fires back at Trump: "I will surely and definitely tame the mentally deranged U.S. dotard with fire." (via KCNA) pic.twitter.com/wZVdg5Q5Iq — Jon Passantino (@passantino) September 21, 2017

“It could be the most powerful detonation of an H-bomb in the Pacific. We have no idea about what actions could be taken as it will be ordered by leader Kim Jong-un,” said North Korean Foreign Minister Ri Yong-ho, according to Yonhap News.

Some analysts have debated what such a launch would look like, with some translations saying a potential detonation would happen “over” the Pacific rather than “in” the ocean. An atmospheric nuclear test has not happened since China conducted one in 1980.

Vipin Narang, a professor of international relations at MIT, said the U.S. should take Kim’s threats “very seriously,” calling such a statement “unprecedented.”

“The actions he is considering, since he believes President Trump only responds to action and doesn’t listen to words, could range from extended-range ICBM or SLBM tests that overfly Japan again or, as the foreign minister suggested, a full-blown live atmospheric nuclear test atop a missile,” Narang said. “An atmospheric nuclear test ... would be a huge provocation. We are talking about putting a live nuclear warhead on a missile that has only been tested a handful of times and overflying potentially populated areas.

“The very prospect is pretty terrifying.”

Pyongyang and Washington have traded escalating threats in recent months, with Trump vowing in August to use “fire and fury” if necessary. His administration has made it clear that military intervention remains on the table as a potential strategy to rein in North Korea.

The Kim dynasty has threatened to rain down its own “fire” against its enemies for decades, including pledges to reduce Seoul “to ashes.” But analysts say Kim’s direct address to a foreign leader is unique, and they fear what he may test next to flex the north’s military might.

It's a statement from Kim himself. He's not backing down. https://t.co/rn3QgeAga3 — Jeffrey Lewis (@ArmsControlWonk) September 21, 2017

I can't think of another time when the North Korean leader has reacted so directly to a U.S. president, let alone with a photo like this pic.twitter.com/YWc8KPChuG — Anna Fifield (@annafifield) September 21, 2017

North Korea has rapidly escalated its antagonistic military agenda this year, prompting widespread international condemnation. The country conducted its sixth and most powerful nuclear test on Sept. 3, claiming it had detonated a hydrogen bomb designed to be carried on an intercontinental ballistic missile. Experts believe it was approximately 17 times as strong as the bomb used to devastate the Japanese city of Hiroshima during World War II. Less than two weeks later, the north also launched a missile over the Japanese mainland, the second time it had done so.

The U.N. Security Council passed its toughest-ever sanctions on North Korea after the test, which U.S. Secretary of State Rex Tillerson later said should “represent the floor, not the ceiling, of the action we should take.”

Trump expanded the sanctions Thursday, targeting financial institutions that conduct business with Pyongyang.

This article has been updated with additional comment on North Korea’s nuclear test threat.