The US Defence Secretary James Mattis has warned North Korea it will be met with a "massive military response" if it provokes the US or any of its territories or allies.
Mr Mattis issued the threat following North Korea's alleged successful test of an advanced hydrogen bomb for a long-range missile.
"Any threat to the United States towards territories including Guam or our allies will be met with a massive military response," he said outside the White House.
"A response both effective and overwhelming."
Mr Mattis says the US' commitment to its allies is "ironclad".
"We have many military options and the President wanted to be briefed on each one of them," he added.
"We made clear we have the ability to defend ourselves and our allies South Korea and Japan from any attack.
"We are not looking to the total annihilation of a country, namely North Korea, but as I said we have many options to do so."
North Korea could 'vaporise' South Korea - UK
Despite the US' strong stance, UK Foreign Minister Boris Johnson has advised caution with military action.
"The distance between North Korea and Seoul (South Korea's capital) is very, very small," Mr Johnson warned.
"[North Korea] could basically vaporise large parts of the South Korean population even with conventional weapons."
He says the UK believes there is still time for diplomacy.
"It is our view in the UK, overwhelmingly, that peaceful diplomatic means are the best [way forward]" Mr Johnson added.
"China is responsible for 90 percent of North Korea's trade and North Korea has only six months of oil supplies left, there is scope to continue to put pressure on the regime."