WASHINGTON >> President Donald Trump and his top national security aides delivered contrasting messages of alarm and reassurance over North Korea’s expanding nuclear capabilities, with the commander in chief touting America’s atomic supremacy a day after threatening “fire and fury” for the communist country.

As international alarm escalated over the still-remote possibility of nuclear confrontation, Trump today dug in on his threats of military action and posted video of his ultimatum to North Korean leader Kim Jong Un. In a rare flexing of America’s own nuclear might, Trump said his first order as president was to “renovate and modernize” an arsenal that is “now far stronger and more powerful than ever before.”

The suggestion that Trump has done anything to enhance U.S. nuclear firepower was immediately disputed by experts, who noted no progress under Trump’s presidency. Still, Trump tweeted: “Hopefully we will never have to use this power, but there will never be a time that we are not the most powerful nation in the world!”

The tweets did little to soothe concerns in the United States and beyond that Trump was helping push the standoff with North Korea into unchartered and even more dangerous territory. While the prospect of military action by either side appears slim, given the level of devastation that would ensue, Trump’s talk Tuesday of “fire and fury like the world has never seen” compounded fears of an accident or misunderstanding leading the nuclear-armed nations into conflict.

This week, an official Japanese report and a classified U.S. intelligence document, as reported by The Washington Post, combined to suggest that the North was closer to being able to strike the United States with a nuclear missile than previously believed. The U.S. document reportedly assessed that the North had mastered the ability to fit a nuclear warhead on its long-range missiles.

After North Korea issued its own warning to the U.S., suggesting it could attack the American territory of Guam, Secretary of State Rex Tillerson sought to calm the sense of crisis.

Speaking earlier today on his way home from Asia, he credited Trump with sending a strong message to the North Korean leader on the “unquestionable” U.S. ability to defend itself, so as to prevent “any miscalculation.” Tillerson insisted the U.S. isn’t signaling a move toward military action, while it pursues a policy of sanctions and isolation of North Korea.

“Americans should sleep well at night,” Tillerson told reporters. He added: “Nothing that I have seen and nothing that I know of would indicate that the situation has dramatically changed in the last 24 hours.”

No sooner had Tillerson ratcheted down the rhetoric than Defense Secretary Jim Mattis ratcheted it back up.

Echoing Trump’s martial tone, Mattis said North Korea should stand down its nuclear weapons program and “cease any consideration of actions that would lead to the end of its regime and the destruction of its people.” As seldom as it is for a president to speak of using nuclear missiles, the reference to the “destruction” of a foreign people is equally rare.

It was unclear, however, how serious to take all the war talk. Markets weren’t rattled by the back-and-forth threats. Trump had no meetings on his schedule today. There were no indications from the Pentagon of urgent planning or new assets being hastily deployed to the Pacific region. And Tillerson even made a pre-scheduled refueling stop in Guam, the target of the North’s purported military designs.

The “fire and fury” proclamation that Trump delivered at his New Jersey golf club was his own message, his spokeswoman said today. It came after Trump and his chief of staff, retired Gen. John Kelly, had been in conversations with members of the National Security Council.

“The tone and strength of the message were discussed beforehand” with advisers, White House press secretary Sarah Huckabee Sanders said. But she said: “The words were his own.”

Trump didn’t mention the words “fire and fury” in a conference call with advisers, according to two officials familiar with the discussion. And Kelly and the other aides didn’t know he would use such vivid terms, said the officials, who weren’t authorized to speak publicly on the matter and demanded anonymity.

The State Department said Tillerson spoke to Trump after he made the comment, a conversation that lasted for about an hour.

Trump’s alarmist tone sparked criticism among his political opponents in the United States and concern among allies and partners in Asia — the very nations the United States would need to work with to avert a military showdown with North Korea.

Senate Minority Leader Chuck Schumer said “reckless rhetoric is not a strategy to keep America safe.” Fellow Democratic Sen. Dianne Feinstein said the situation needs diplomacy, “not saber rattling.” China, North Korea’s increasingly alienated ally, cautioned all sides against “employing words or actions that could sharpen differences and escalate the situation.”

Confusion in the administration’s message has reigned for weeks.

Tillerson has asserted repeatedly the U.S. isn’t seeking “regime change” in North Korea, urging an eventual dialogue. Other Trump aides have been more equivocal. Vice President Mike Pence last week rejected the notion of direct talks with North Korea; CIA Director Mike Pompeo last month claimed North Korea’s people would love to see their leader’s back.

The North Koreans, meanwhile, deployed their own, typically febrile rhetoric.

On Tuesday, the North Korean army said it’s studying a plan to create an “enveloping fire” around Guam with medium-range and long-range ballistic missiles.

The territory lies about 2,100 miles from the Korean Peninsula, and it’s extremely unlikely Kim’s government would risk its assured annihilation with a pre-emptive attack on U.S. citizens. It’s also unclear how reliable North Korea’s missiles would be against such a distant target, but no one was writing off the danger completely.

One regular critic of Trump’s foreign policy stood up for the president. Sen. Lindsey Graham, a Republican hawk, said he felt Trump wasn’t bluffing with his threat of “fire and fury.” Trump has “basically drawn a red line” by saying Pyongyang can’t ever have a nuclear-tipped missile capable of striking the U.S., he said.

Trump loyalists rejected criticism of their president.

“These are the moments when we have to come together as a nation and support the executive, whether you voted for him or not,” Sebastian Gorka, a Trump adviser on national security issues, told Fox News.

Likening the situation to the 1962 Cuban missile crisis that almost plunged the U.S. and Soviet Union into nuclear war, Gorka accused Trump’s critics of being unpatriotic.

Associated Press writers Foster Klug and Kim Tong-hyung and Hyung-jin Kim in Seoul, Christopher Bodeen in Beijing, Deb Riechmann and Lolita C. Baldor in Washington, and Jonathan Lemire in New York contributed to this report.