The UN Security Council is to hold an emergency meeting to discuss an international response to North Korea's nuclear test. Calls for tougher sanctions have come from Europe, the US and Japan.
North Korea said Sunday it successfully tested a hydrogen bomb that can be placed on an intercontinental ballistic missile. The state news agency made the announcement after geological agencies registered a man-made quake in the northeast of the country.
The South Korean military said it was analyzing a "presumed" nuclear test, while Japan said it had determined North Korea conducted a nuclear test.
Past North Korean nuclear tests have resulted in earthquakes, but a 6.3 magnitude tremor indicates that the size of the nuclear test was larger than previous ones, analysts said.
South Korea's Meteorological Administration estimated that Sunday's test was five to six times more powerful than North Korea's fifth nuclear test in September 2016, which caused a 5.3 magnitude earthquake. It said it was about 10 times stronger than a nuclear test in January 2016.
The nuclear test was widely condemned, with South Korea and Japan both calling for new UN sanctions on North Korea.
German Chancellor Angela Merkel said North Korea's provocations had "reached a new dimension" with the latest nuclear test. Merkel spoke on the phone with French President Emmanuel Macron. Both leaders condemned North Korea and expressed their support for a tightening of EU sanctions against it.
China, North Korea's only ally, condemned the nuclear test as the "wrong actions." The United States has repeatedly pressed Beijing to do more to rein in its neighbor. Russia also condemend the nuclear test, but urged all sides to refrain from taking action that could escalate tensions.
US President Donald Trump, who earlier this week said there was little room for diplomacy, repeated his criticism of China and labelled South Korea, a US treaty ally, of "appeasement."
In China for the BRICS meeting, Russian President Vladimir Putin called for restraint in response to the test. Kremlin spokesman Dmitry Peskov told journalists that Putin "said the international community could not give in to emotions, should act calmly and deliberately, and stressed that the complex settlement of the nuclear and other problems of the Korean Peninsula can be achieved exclusively through political and diplomatic means."
President of the European Commission Donald Tusk said the UN Security Council should "adopt further UN sanctions and show stronger resolve to achieve a peaceful denuclearization of the Korean peninsula," adding "The stakes are getting too high."
North Korea announces new H-bomb
Hours before Sunday's test, Pyongyang announced it had developed a thermonuclear weapon with "super explosive power," the official Korean Central News Agency (KCNA) claimed, citing leader Kim Jong-Un as saying "all components of the H-bomb were 100 percent domestically made."
The KCNA said Kim had inspected such a device at the Nuclear Weapons Institute, with pictures showing him in a black suit examining a metal casing.
North Korea has "further upgraded its technical performance at a higher ultra-modern level on the basis of previous successes made in the first H-bomb test," the KCNA said.
There was no independent confirmation Sunday's test was a hydrogen bomb instead of a less powerful atomic bomb, but the size of the nuclear test raised further concerns over North Korea's nuclear progress.
"If confirmed as a nuclear test, this act would indicate that the DPRK's (North Korea) nuclear program is advancing rapidly," said Lassina Zerbo, the head of the Comprehensive Test Ban Treaty Organisation.
Pyongyang triggered a new escalation of tensions in July after it carried out two successful tests of an intercontinental ballistic missile , the Hwasong-14, bringing much of the US mainland within range. It later said it would conduct ballistic missile tests near Guam, the location of a US military base.
Last week, North Korea fired a ballistic missile over Japan and into the Pacific Ocean.
Japan has also called for a concerted international effort to put an end to the "growing threat" posed by North Korea's nuclear program.
Questions remain over whether Pyongyang has been able to develop a miniturized nuclear device that would be light enough to be placed on an intercontinental ballistic missile without impacting its range and reentry into the Earth's atmosphere.
In January 2016, Pyongyang claimed the device used - its fourth test - was a miniaturized H-bomb. Scientists believe the six-kiloton yield achieved then was too low for a thermonuclear device.
When it carried out its fifth test, in September 2016, it backed away from earlier claims of having tested a hydrogen bomb.
Repeated rounds of international sanctions on North Korea have failed to dent the progress of the country's nuclear and ballistic missile programs.