North Korea has detonated a hydrogen bomb with "perfect success", the country's state media has announced.
It claimed the device could be mounted on its newly developed intercontinental ballistic missiles, which experts have said are capable of reaching the US mainland.
Pyongyang's only major ally, China, said it strongly condemned the detonation.
Earlier, Japan confirmed its near neighbour had conducted a sixth nuclear test.
The country's meteorological agency said the resulting tremors were at least 10 times as powerful as North Korea's previous nuclear test, last September.
Experts estimated that blast to have been around 10 kilotons.
It means this latest device was about five times larger than the bomb dropped on the Japanese city of Nagasaki in World War II.
The US Geological Survey said a magnitude 6.3 tremor struck near a weapons test site in the northeast of North Korea.
A second tremor measuring 4.6 was also detected, South Korea's Yonhap news agency said.
Japan's foreign minister, Taro Kono, described the new explosion as "extremely unforgivable".
The Tokyo government has registered a protest with the North Korean embassy in Beijing, he said.
Hours earlier, Japanese Prime Minister Shinzo Abe spoke to US President Donald Trump on the phone about the "escalating" situation.
South Korea has called for the "strongest possible" response, including new sanctions from the UN Security Council to "completely isolate" its northern neighbour.
Seoul's national security adviser, Chung Eui-yong, said there had been talks with Washington about deploying US strategic military assets to the Korean peninsula.
Sky News' Asia Correspondent, Katie Stallard, said: "The significant point to take out of all of this is in terms of what response we see from the United States.
"We've seen over the last weeks and months increasing rhetoric from both sides, culminating memorably in Donald Trump's threat of 'fire and fury' against Kim Jong Un's regime if they continued to make threats.
"So the question is, given his previous rhetoric, his previous threats, how Donald Trump plans to respond to this."
Sky News' Diplomatic Editor, Dominic Waghorn, said Mr Trump had to respond "one way or another, so there's a real danger here of provocation, escalation, miscalculation, leading to something really quite devastating in that part of the world".
The blast came just hours after Pyongyang
claimed to have developed a hydrogen bomb that could be loaded into a long-range missile.
The state-run Korean Central News Agency released pictures of Kim Jong Un visiting the country's Nuclear Weapons Institute where he inspected the purported device.