North Korea said Sunday it successfully tested a hydrogen bomb that can be placed on an intercontinental ballistic missile.
The announcement comes 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.
The United States Geological Survey said the 6.3 magnitude earthquake was a "possible explosion, located near the site where North Korea has detonated nuclear explosions in the past."
China's earthquake agency said it had detected a 6.3 magnitude earthquake in Kilju, Hamgyong Province that was a "suspected explosion." It said it detected a second 4.6 magnitude "cave in" minutes after the first, but South Korea's weather agency said it did not detect a second tremor.
Past North Korean nuclear tests have resulted in earthquakes, but a 6.3 magnitude tremor indicates that the size of this suspected nuclear test was larger than previous ones, analysts said.
North Korea announces new H-bomb
The quake came shortly after 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.
Pyongyang triggered a new escalation of tensions in July after it carried out two successful tests of an intercontinental ballistic missile (ICBM), the Hwasong-14, bringing much of the US mainland within range. Japan has also called for a concerted international effort to put an end to the "growing threat" posed by North Korea's nuclear program.
US President Donald Trump spoke by telephone with Japanese Prime Minister Shinzo Abe following the North Korean announcement, the White House said.
"We completely agreed that we must thoroughly coordinate with each other and with South Korea, and cooperate closely with the international community, to increase pressure on North Korea and make it change its policies," Abe told reporters
Trump and Moon also discussed North Korea's "continued destabilizing and escalatory behavior," the White House said in a statement.
Trump has warned that the US military is "locked and loaded" and that North Korea would face "fire and fury" in the event of further provocation. North Korea said the test fire of a missile that flew over Japan was a "curtain-raiser" for its "resolute countermeasures" against ongoing US-South Korean military drills.
"Though we cannot verify the claim, [North Korea] wants us to believe that it can launch a thermonuclear strike now, if it is attacked," Adam Mount, a senior fellow at the Center for American Progress, told Reuters news agency.
Pyongyang's assertion that "this warhead is variable-yield and capable of specialized weapons effects implies a complex nuclear strategy," Mount added. "It shows [North Korea] is not only threatening assured destruction of the US and allied cities in the event it is attacked, but also is considering limited coercive nuclear strikes, or is seeking credible response options for US ones."
Questions remain over whether Pyongyang has been able to develop a miniturized nuclear device that is small and light enough to be placed on an ICBM 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.
(This article was first published on DW. You can read the original article here.)