North Korea declares itself a thermonuclear power, after carrying out a sixth nuclear test, which Japanese and South Korean officials say was several times more powerful than the tremor picked up after its last nuclear test a year ago.
Countries and international bodies around the world swiftly condemned North Korea's announcement that it had tested a hydrogen bomb on Sunday.
Pyongyang, for its part, called the nuclear detonation "a perfect success".
Tensions on the Korean peninsula have spiralled in recent weeks, with North Korea testing an intercontinental ballistic missile (ICBM) and threatening to fire missiles towards the US Pacific island of Guam and President Donald Trump warning he would rain "fire and fury" on the country.
Chinese President Xi Jinping and Russian President Vladimir Putin agreed Sunday to "appropriately deal with" the latest nuclear test by North Korea, state news agency Xinhua said.
"The two leaders agreed to stick to the goal of denuclearisation on the Korean Peninsula and keep close communication and coordination to deal with the new situation," Xinhua said in a brief dispatch.
China, which is hosting a summit of the five BRICS nations, said it "expresses resolute opposition and strong condemnation" over Pyongyang's sixth nuclear test, which was felt in Chinese cities hundreds of kilometres from North Korea's borders.
EU foreign policy chief Federica Mogherini said she was looking forward "to the UN Security Council addressing the matter and taking a firm and effective stand".
"The international community must be very firm in its handling of this latest provocation, in order to get North Korea to unconditionally return to talks and proceed with the complete, verifiable and irreversible dismantlement of its nuclear and ballistic programmes," French President Emmanuel Macron said in a statement.
He called on the UN Security Council to "quickly react" to the escalation and also urged the EU to come up with a "clear and united" response.
The German government said in a statement that Merkel and Macron spoke on the phone, with both strongly condemning the new nuclear test in North Korea. “This latest provocation by the ruler in Pyongyang has reached a new dimension,” it added.
US President Donald Trump and Japan's Prime Minister Shinzo Abe agreed that the international community must respond to North Korea with greater pressure than before, Japanese news agency Kyodo said.
Abe slammed the nuclear test earlier as "absolutely unacceptable" and said its nuclear and missile programmes now pose a more "grave and urgent" threat to his country.
"(North Korea) ignored repeated warnings by the international community and forcibly conducted a nuclear test. It is absolutely unacceptable," Prime Minister Shinzo Abe told reporters.
"Whether we can stop North Korea's reckless actions that threaten world peace depends on the cooperation and solidarity of the international community," he added.
International Atomic Energy Agency
The head of the UN's nuclear watchdog, IAEA Director General Yukiya Amano said the nuclear test was "an extremely regrettable act" that was "in complete disregard of the repeated demands of the international community".
NATO secretary-general Jens Stoltenberg condemned the test as "yet another flagrant violation" of multiple UN Security Council resolutions.
I strongly condemn North Korea's aggressive missile test-flight over Japan. North-Korea must stop its missile programs.— Jens Stoltenberg (@jensstoltenberg) August 29, 2017
Russia urged calm on Sunday as it slammed North Korea testing a hydrogen bomb.
"This latest demonstrative disregard by Pyongyang of the requirements of the relevant resolutions of the UN Security Council and the norms of international law deserves the strongest condemnation," the foreign ministry in Moscow said.
The ministry said it regretted that the leadership of North Korea was "creating a serious threat" for the region and warned that "the continuation of such a line is fraught with serious consequences" for Pyongyang.
"In the unfolding conditions, it is imperative to remain calm and to refrain from any actions that lead to a further escalation of tension," a statement said.
South Korean President Moon Jae-In called for the "strongest punishment" against the North over its nuclear test, including new UN sanctions to "completely isolate" it.
"I can't help but be disappointed and outraged," a handout from the president's office said.
"North Korea had made an absurd tactical mistake by committing a series of provocations such as launching the ICBM missiles and conducting a nuclear test, which was heightened tensions on the peninsula and is threatening world peace. It will isolate them further."
British Prime Minister Theresa May said the United Nations Security Council should urgently look at imposing new sanctions on North Korea 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," May said in an emailed statement.
"I discussed the serious and grave threat these dangerous and illegal actions present with President 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."
Trump responded on Twitter, calling North Korea's actions "very hostile and dangerous" to the United States.
North Korea has conducted a major Nuclear Test. Their words and actions continue to be very hostile and dangerous to the United States.....— Donald J. Trump (@realDonaldTrump) September 3, 2017
..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.— Donald J. Trump (@realDonaldTrump) September 3, 2017
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!— Donald J. Trump (@realDonaldTrump) September 3, 2017
US Treasury Secretary Steve Mnuchin said he would put together a package of new sanctions to potentially cut off all trade with North Korea after Pyongyang conducted its most powerful nuclear test.
Speaking on Fox News, Mnuchin said he would ask Trump to strongly consider cutting off all trade with new sanctions.
Trump will convene his national security team on Sunday, the White House said.