Pentagon chief James Mattis said any threat to the US or its allies by North Korea will be met with a “massive military response”.
His comments on Sunday came after a national security briefing with President Donald Trump about the secretive communist state’s latest nuclear test, BBC reported.
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.
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 US or its territories – including Guam – or our allies will be met with a massive military response, a response both effective and overwhelming.”
However, he said the hope was for denuclearisation, “because we are not looking to the total annihilation of a country, namely North Korea”.
The UN Security Council is to hold an emergency meeting on Monday to discuss an international response, according to the US mission.
Meanwhile, President Trump has warned that America may stop trading with any country that does business with the North.
The first suggestion that this was to be a far from normal Sunday in the region came when seismologists’ equipment started picking up readings of an earth tremor in the area where North Korea has conducted nuclear tests before.
The US Geological Survey put the tremor at 6.3 magnitude.
Japanese Foreign Minister Taro Kono said there was no doubt this was North Korea’s sixth nuclear test, calling it “unforgivable”.
Then North Korean state media confirmed this was no earthquake.
It claimed the country had conducted its sixth and most powerful nuclear test, detonating a hydrogen bomb that could be loaded onto a long-range missile.
North Korean leader Kim Jong-un was pictured with what state media said was a new type of hydrogen bomb.
Hydrogen bombs are many times more powerful than an atomic bomb.
They use fusion – the merging of atoms – to unleash huge amounts of energy, whereas atomic bombs use nuclear fission, or the splitting of atoms.
Analysts say the North’s claims should be treated with caution, but that its nuclear capability is clearly advancing.
Officials in China, where the blast was felt as a tremor, said they were carrying out emergency radiation testing along the border with North Korea.