Pentagon chief James Mattis says any threat to the US or its allies by North Korea will be met with a "massive military response".
His comments came after a national security briefing with President Donald Trump about the secretive communist state's latest nuclear test.
Pyongyang says it has successfully trialled a hydrogen bomb that could be loaded on to a long-range missile.
The move has drawn international condemnation.
North Korea has defied UN sanctions and international pressure by developing nuclear weapons and test missiles that could potentially reach the US.
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But speaking to reporters outside the White House, Defence Secretary Mattis said the US had the ability to defend itself and its allies South Korea and Japan, adding that its commitments were "ironclad".
"Any threat to the United States or its territories - including Guam - or our allies will be met with a massive military response, a response both effective and overwhelming."
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