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.
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.”
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 NorthWhat has happened?
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.
What has the reaction been?
Denouncing the test as “hostile” and “dangerous”, President Trump described the North as a “rogue nation” which had become a “great threat and embarrassment” to China – Pyongyang’s main ally.
He also said South Korea’s “talk of appeasement” was not working and that the secretive communist state “only understands one thing”.
“The United States is considering, in addition to other options, stopping all trade with any country doing business with North Korea,” Mr Trump later said in a tweet. North Korea relies on China for about 90% of its foreign trade.
South Korean President Moon Jae-in called for the “strongest possible” response, including new UN Security Council sanctions to “completely isolate” the country.
China, meanwhile, also expressed “strong condemnation” and said the state “had ignored the international community’s widespread opposition”.
Russia urged all sides involved to hold talks, saying this was the only way to resolve the Korean peninsula’s problems.
UK Prime Minister Theresa May said the “reckless” new test represented an “unacceptable further threat to the international community”. She called on world leaders to come together to stop North Korea’s “destabilising actions”.
What does the test tell us?
South Korean officials said the latest test took place in Kilju County, where the North’s Punggye-ri nuclear test site is situated. The “artificial quake” was 9.8 times more powerful than the tremor from the North’s fifth test in September 2016, the state weather agency said.
Although experts urged caution, this does appear to be the biggest and most successful nuclear test by North Korea to date – and the messaging is clear. North Korea wants to demonstrate it knows what makes a credible nuclear warhead.
Nuclear weapons expert Catherine Dill told the BBC it was not yet clear exactly what nuclear weapon design was tested.
“But based on the seismic signature, the yield of this test definitely is an order of magnitude higher than the yields of the previous tests.”
Current information did not definitively indicate that a thermonuclear weapon had been tested “but it appears to be a likely possibility at this point”, she said
What can be done?