Donald Trump was speaking in his own words when he vowed to retaliate against North Korean aggression with "fire and fury", the White House has said.
Aides said the US president's blunt warning was largely spontaneous, but reflected his growing anger with Pyongyang's rising nuclear capabilities.
Mr Trump's apparent willingness to improvise marks a sharp departure from the carefully calculated approach past administrations have taken to countering years of over-the-top threats from North Korea and other delicate diplomatic matters.
His stunning rhetoric, which suggested a military clash between two nuclear-armed nations, also underscored that the president is willing to apply his unconventional and unpredictable approach to governing to the international crises that cross his desk.
American presidents have long talked tough about US military prowess in confrontations with foreign foes.
Bill Clinton stood on the border between North and South Korea to declare that if Pyongyang deployed nuclear weapons "it could be the end of their country" and George W Bush put North Korea in the "axis of evil" - a trio of nations that included Iraq, which the US invaded the next year.
But Mr Trump's words, and his apparent spontaneous bluster, still stood out.
The president huddled with advisers on Tuesday to discuss how he would address North Korea's provocations.
White House spokeswoman Sarah Huckabee Sanders said the "tone and strength of the message were discussed" but that the exact phrasing was "his own".
Three officials familiar with the discussion said Mr Trump did not mention the words "fire and fury" during his discussion with new chief of staff John Kelly and others.
He indicated that he wanted to strike a tough tone, but did not mention specific language, according to the officials.
The White House account left Mr Trump responsible for the escalatory rhetoric and left his party divided on his approach.
"The great leaders I've seen don't threaten unless they're ready to act and I'm not sure President Trump is ready to act," said Republican senator John McCain.
Senator Lindsey Graham of South Carolina said Mr Trump had "basically drawn a red line" and declared he was not going to just "contain the threat. He's going to stop the threat".
The Trump administration has experimented with different approaches to North Korea.
At times, the White House has tried to deprive Pyongyang of the attention experts believe its erratic leader seeks with each nuclear test.
But as the North's capacity has continued to increase, Mr Trump and some of his advisers have stepped in with increasingly alarmist rhetoric.
Some national security experts said Mr Trump was confronting more challenging circumstances than his predecessors, given the advances made by the North in developing its nuclear weapon and missile programmes.
"All of the previous presidents faced the theoretical possibility of Korea having a nuclear weapon that it could put on a missile and the missile reaching the United States," said Anthony Cordesman, an analyst at the Centre for Strategic and International Studies.
"Traditional diplomacy is not designed to deal with threats of nuclear attacks and it is not designed to deal with the repeated defiance of the United Nations."
The president's fiery rhetoric came as tensions mounted between the US and North Korea.
After Mr Trump's "fire and fury" comment, Pyongyang announced it was examining plans for attacking the tiny Pacific island of Guam.
The escalating threats followed reports that North Korea has mastered a crucial technology needed to strike the United States with a nuclear missile.
The United Nations Security Council has adopted new, tougher sanctions, seeking to curtail North Korea's nuclear ambitions.
But not everyone in the administration was following Mr Trump's lead.
Secretary of state Rex Tillerson, who made a refuelling stop in Guam on Wednesday following a trip to Asia, said Americans should "have no concerns about this particular rhetoric of the last few days" and "Americans should sleep well at night".
Jim Carafano, a Heritage Foundation scholar who advised Mr Trump's transition team on foreign policy, said the messages conveyed to North Korea were aimed at different audiences: Mr Trump to the American people, Mr Tillerson to the diplomatic community and Mr Mattis to the military establishment.
By using stark terms, Mr Carafano said Mr Trump aimed to tell "the people who put him into office that you voted for me to be a tough commander-in-chief, and I will defend the nation".