North Korea says it has 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."
"South Korea is finding, as I have told them, that their talk of appeasement with North Korea will not work, they only understand one thing!" he wrote.
He later 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.
Mr Trump and Japanese Prime Minister Shinzo Abe agreed the international community must step up its response, while Mr Abe also said he and Putin would cooperate.
Mr 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.
US President Donald Trump tweeted "South Korea is finding, as I have told them, that their talk of appeasement with North Korea will not work, they (North Korea) only understand one thing!"
In another tweet he said North Korea's "words and actions continue to be very hostile and dangerous to the United States" and the regime "has become a great threat and embarrassment to China, which is trying to help but with little success."
Later when asked by reporters if the US would attack North Korea the president said: "we'll see".
President Emmanuel Macron: "The international community must treat this new provocation with the utmost firmness, in order to bring North Korea to come back unconditionally to the path of dialogue and to proceed to the complete, verifiable and irreversible dismantling of its nuclear and ballistic program."
China, the only North Korean ally that is a permanent member of the UN Security Council, urged its neighbour to stop "wrong" actions that worsen the situation. It said it would fully enforce UN resolutions on the country.
The Russian foreign ministry: "In the emerging conditions it is absolutely essential to keep cool, refrain from any actions that could lead to a further escalation of tensions," it said on its website, adding that North Korea risked "serious consequences".
Japan's Abe said North Korea's "nuclear and missile development programs pose a new level of a grave and immediate threat" and "seriously undermines the peace and security of the region".
South Korean President Moon Jae-in said North Korea's sixth nuclear test should be met with the "strongest possible" response, including new United Nations Security Council sanctions to "completely isolate" the country.
Australia has condemned North Korea's "flagrant defiance" of UN Security Council resolutions and urged the world body to take further action against the "dangerous pariah regime".
"We call for the UN Security Council to urgently consider further strong measures that would place pressure on North Korea to change course," Prime Minister Malcolm Turnbull said in a joint statement with Foreign Minister Julie Bishop.
Australia called for all countries, especially the five UN veto powers "to apply the maximum possible pressure to this dangerous pariah regime", according to the statement.
British Prime Minister Theresa May said the United Nations Security Council should urgently look at imposing new sanctions and speed up implementation of existing ones.
"This latest action by North Korea is reckless and poses an unacceptable further threat to the international community," Ms May said in an emailed statement.
"I discussed the serious and grave threat these dangerous and illegal actions present with Prime Minister Abe in Japan this week and reiterate the call we jointly made for tougher action, including increasing the pace of implementation of existing sanctions and looking urgently in the UN Security Council at new measures."
Britain's foreign minister Boris Johnson called the nuclear test "reckless" and a "provocation".
German Chancellor Angela Merkel said North Korea's provocations have "reached a new dimension" with the nation's sixth nuclear test.
Merkel spoke on the phone Sunday with France's Macron. Her office said both leaders "condemn North Korea's new nuclear tests in the sharpest possible terms".
The International Atomic Energy Agency, which has no access to North Korea, called the nuclear test, Pyongyang's test is "an extremely regrettable act" that is "in complete disregard of the repeated demands of the international community".