The US has joined condemnation of North Korea after it tested what it says is a more advanced nuclear weapon.
Pyongyang said it had successfully trialled a hydrogen bomb that could be loaded on to a long-range missile.
President Donald Trump tweeted that North Korea’s “words and actions” were “very hostile and dangerous”.
North Korea has defied UN sanctions and international pressure to develop nuclear weapons and test missiles that could potentially reach the US.
South Korea, China and Russia have all voiced strong criticism of the North’s sixth nuclear test.
Responding to what he called a “major nuclear test” on Sunday, Trump said South Korea’s “talk of appeasement” was not working and that the secretive communist state “only understands one thing”.
“North Korea is a rogue nation which has become a great threat and embarrassment to China, which is trying to help but with little success,” he added.
Meanwhile, South Korean President Moon Jae-in called for the “strongest possible” response, including new UN Security Council sanctions to “completely isolate” the country.
“I can’t help but be disappointed and outraged,” he said, adding that North Korea’s weapons programme was “threatening world peace” and would only “isolate them further”.
China, North Korea’s only major ally, also expressed “strong condemnation” and said the state “had ignored the international community’s widespread opposition”.
Meanwhile, Russia urged all sides involved to hold talks, saying this was the only way to resolve the Korean peninsula’s problems.
North Korea last carried out a nuclear test in September 2016.
The International Atomic Energy Agency (IAEA), the global nuclear watchdog, described the new test as “an extremely regrettable act”, which was “in complete disregard” of the demands of the international community.
UK Foreign Minister Boris Johnson denounced the “reckless” test and warned that being able to fit a warhead to a missile would present a “new order of threat” from Kim Jong-un’s regime.
North Korea claimed its sixth nuclear test was a “perfect success”, after seismologists had detected an earth tremor.
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, the state weather agency said.
It came hours after Pyongyang said it had miniaturised a hydrogen bomb – which is much more powerful than an atomic bomb – for use on a long-range missile. North Korean leader Kim Jong-un was pictured with what state media said was a new type of hydrogen bomb. State media said the device could be loaded on to a ballistic missile.