By RFE/RL

North Korea says it has successfully tested a hydrogen bomb capable of being loaded onto its long-range missiles -- in what was apparently its most powerful nuclear test yet.

State media said on September 3 that Pyongyang's sixth nuclear test was a "perfect success" and was a "meaningful" step in completing the country's nuclear weapons program.

There was no independent confirmation that the detonation was a hydrogen bomb, which is more powerful than a regular atomic bomb -- like the ones the United States dropped on Japan near the end of World War II.

However, Japan’s government confirmed that the reclusive nation had carried out a nuclear test after seismologists detected a powerful earthquake in an area where North Korea had conducted previous tests.

Pyongyang has conducted nuclear tests since 2006 in violation of United Nations resolutions, igniting furious condemnation from the United States and most other nations worldwide.

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In January 2016, North Korea claimed it had successfully tested a hydrogen bomb, but Western experts doubted at the time that it had the power of a thermonuclear explosion.

Following Pyongyang’s latest test, South Korea held a National Security Council meeting chaired by President Moon Jae-in.

National Security Director Chung Eui-yong said Seoul will seek every available measure, including new UN sanctions, to further isolate Pyongyang.

National Security Director Chung Eui-yong

he added.

Japan's Prime Minister Shinzo Abe called a test "absolutely unacceptable."

U.S. National Security Adviser H.R. McMaster held a telephone call with the director-general of the Japanese National Security Council, Shotarou Taniuchi, according to Japan’s government.

McMaster made the assurance that Washington is firmly committed to defending Japan, including with its nuclear deterrent, a statement said.

China's Foreign Ministry urged North Korea to stop its "wrong" actions and urged the country to respect UN Security Council resolutions.

Russia said the nuclear test “deserves the strongest condemnation" and called on all interested parties to “immediately return to dialogue.”

a Foreign Ministry statement said.

French President Emmanuel Macron called for a "very firm" response by the international community to North Korea's “latest provocation."

The director-general of the UN atomic watchdog, the Vienna-based International Atomic Energy Agency (IAEA), said North Korea's latest nuclear test was "extremely regrettable" and "in complete disregard" of the international community's repeated demands.

The quake was detected hours after North Korean state media raised tensions by claiming the country had successfully developed an advanced hydrogen bomb that could be loaded onto an intercontinental ballistic missile (ICBM).

Neither claim could be independently confirmed.

The official KCNA news agency said the hydrogen bomb was the country’s most-advanced nuclear weapon and has "great destructive power."

Following that announcement, the U.S. Geological Survey said it detected an earthquake of 6.3 magnitude at a depth of zero kilometers. Jana Pursely, a USGS geophysicist, told the AFP news agency that "it's an explosion rather than an earthquake."

The Chinese Earthquake Administration described the quake as a "suspected explosion." It detected a second quake minutes after the first, saying that it might be a "collapse earthquake," suggesting the rock over the underground blast had given way.

South Korean officials said that the “artificial earthquake” took place in Kilju County, where the North's Punggye-ri nuclear test site is situated.

"The scale of the energy was five to six times more powerful” than North Korea’s fifth nuclear test, which occurred in September 2016, according to Lee Mi-Sun, head of the South Korean meteorological agency.

Russian monitors said that radiation levels in the country’s Far East remained "in normal range" after Pyongyang said it tested a hydrogen bomb. "No excesses in background radiation levels were detected,” local state monitoring service Primgidromet said in a statement.

Before the indications of the possible nuclear test, White House and Japanese officials said U.S. President Donald Trump and Japanese Prime Minister Shinzo Abe had spoken by phone in the face of the "escalating" situation with North Korea.

The leaders were quoted as agreeing that close cooperation between their countries and South Korea was needed, along with increased pressure on Pyongyang.

Tensions between Pyongyang and many world powers, particularly the United States, Japan, and South Korea, were already at their highest levels in years.

Trump and Kim have exchanged threats in a war of words that intensified after Pyongyang test-launched two ICBM-class missiles in July that had a potential range of 10,000 kilometers – enough to hit many parts of the United States.

There had been speculation that the North could conduct another nuclear test on September 9, the anniversary of the founding of the country.

The United States and South Korea on September 1 said they agreed to strengthen Seoul's defenses, and Washington approved the sale of billions of dollars worth of arms to the South to counter the North.

Russian President Vladimir Putin, in an article published on September 1, said the standoff over North Korea's nuclear and missile programs is "on the verge of a large-scale conflict."

Putin indirectly criticized U.S. warnings of potential military action and said stepping up pressure on Pyongyang won't solve the problem, adding that the crisis can only be settled through “direct dialogue between all concerned parties, without preconditions."

With reporting by AFP, AP, Reuters, and dpa