North Korea says its tested an advanced hydrogen bomb for a long-range missile, marking a dramatic escalation of the regime's stand-off with the United States and its allies.
State television said the hydrogen bomb test ordered by leader Kim Jong Un had been a "perfect success".
The bomb was designed to be mounted on its newly developed intercontinental ballistic missile.
North Korea's sixth nuclear test drew swift international condemnation.
US President Donald Trump described North Korea as a "dangerous" "rogue nation", later when asked if he would attack the North he said: "We'll see."
He late tweeted he was considering a global trade embargo, which would cut US trade with any country doing business with Pyongyang.
The big question is whether advisers like defence secretary James Mattis and Secretary of State Rex Tillerson can persuade Trump not to abandon diplomacy.
Chinese President Xi Jinping and Russian President Vladimir Putin agreed to "appropriately deal" with the test.
Trump and Japanese Prime Minister Shinzo Abe agreed the international community must step up its response, while Abe also said he and Putin would cooperate.
Putin has no plans to telephone North Korean leader Kim Jong-un, Kremlin spokesman Dmitry Peskov said on Sunday.
The test registered with international seismic agencies as a man-made earthquake.
Japanese and South Korean officials said it was about 10 times more powerful than the tremor picked up after North Korea's last nuclear test a year ago.
There was no independent confirmation the detonation was a hydrogen bomb, but Japan's Chief Cabinet Secretary Yoshihide Suga said Tokyo could not rule out such a possibility.
A US intelligence official, speaking on condition of anonymity, said the US had no reason to doubt it was an advanced nuclear device tested.
Experts who studied the impact of the earthquake caused by the explosion - measured by the US Geological Survey at magnitude 6.3 - said that there was enough strong evidence to suggest the reclusive state has either developed a hydrogen bomb or was getting very close.
South Korean President Moon Jae-in said Seoul would push for strong steps to further isolate the North, including new UN sanctions. Japan also raised the prospect of further sanctions, saying curbs on North Korea's oil trade would be on the table.
China, North Korea's sole major ally, strongly condemned the nuclear test and urged Pyongyang to stop its "wrong" actions.
The US wants urged Beijing to do more to rein in its neighbour, but Beijing rejects that it's solely responsible for doing so. China says military drills by South Korea and the US on the Korean peninsula add to tensions.
Under third-generation leader Kim, North Korea has been pursuing a nuclear device small and light enough to fit on a long-range ballistic missile, without affecting its range and making it capable of surviving re-entry.
North Korea claimed in January last year to have tested a miniaturised hydrogen bomb, also known as a thermonuclear device, but outside experts were sceptical.
