US Defence Secretary Jim Mattis has told North Korea to avoid actions that could usher in the “end of its regime and the destruction of its people”.
The Pentagon chief also said Pyongyang would be “grossly overmatched” in a war against the US and its allies.
His stark warning comes a day after President Donald Trump threatened North Korea with “fire and fury”.
US Secretary of State Rex Tillerson sought to assure Americans there was no imminent threat.
In Wednesday’s strongly worded statement, Mr Mattis called on Pyongyang to halt its arms programme.
“The DPRK (Democratic People’s Republic of Korea) must choose to stop isolating itself and stand down its pursuit of nuclear weapons,” the US Marine Corps general said.
“While our state department is making every effort to resolve this global threat through diplomatic means, it must be noted that the combined allied militaries now possess the most precise, rehearsed and robust defensive and offensive capabilities on Earth.”
Pyongyang’s actions “will continue to be grossly overmatched by ours and would lose any arms race or conflict it initiates”, Mr Mattis added.
Mr Trump – who has said he will not allow Pyongyang to develop a nuclear weapon capable of hitting the US – boasted earlier on Wednesday of America’s atomic arsenal.
In early morning tweets from New Jersey where he is on holiday, the president said the US nuclear stockpile was “more powerful than ever before”.
The tweets come amid escalating rhetoric between the two sides.
North Korea’s state-run KCNA news agency said on Wednesday that Pyongyang was “carefully examining” a plan to strike the US territory of Guam.
The Pacific island is home to US military bases, strategic bombers and about 163,000 people.
Stopping in Guam to refuel after a trip to South East Asia, Mr Tillerson said he was hopeful a global “pressure campaign” involving Russia and China could lead to new dialogue with Pyongyang “about a different future”.
America’s top diplomat also defended President Trump’s language.
He said North Korean leader Kim Jong-un did not understand diplomatic language, and a strong message was needed that he could understand.
Mr Tillerson added the situation had not dramatically changed over the last few days, and that Americans “should sleep well at night”.
China has urged calm, describing the situation as “complex and sensitive”.