WASHINGTON – Raising the stakes of its challenge to the world, North Korea early Monday conducted an underground test of a nuclear weapon far more powerful than its first nuclear test nearly three years ago. The rogue nation also apparently tested at least one short-range missile.

“The Democratic People’s Republic of Korea successfully conducted one more underground nuclear test on May 25 as part of the measures to bolster up its nuclear deterrent for self-defense,” North Korea announced through its state-run news agency.

The agency boasted that its new weapon was more powerful and more technologically advanced than the one North Korea tested in October 2006, calling it “a new higher level in terms of its explosive power and technology of its control. The results of the test helped satisfactorily settle the scientific and technological problems arising in further increasing the power of nuclear weapons and steadily developing nuclear technology.”

The power of the explosion wasn’t immediately clear. Russian officials said it measured 10 to 20 kilotons, or about the size of the atomic bombs that were dropped on Hiroshima and Nagasaki in World War II. U.S. officials estimated the earlier weapon measured less than 1 kiloton. One kiloton represents the force of 1,000 tons of TNT.

It clearly was powerful, however, shaking the earth and felt as far away as the Chinese border city of Yanji, 130 miles from the test site.

The United States and other nations immediately condemned the test.

“North Korea’s nuclear ballistic missile programs pose a great threat to the peace and security of the world, and I strongly condemn their reckless action,” President Barack Obama said at the White House. “North Korea’s actions endanger the people of Northeast Asia, they are a blatant violation of international law, and they contradict North Korea’s own prior commitments.”

Obama said that the United States and other countries must take action in response. He noted that China and Russia, as well as “our traditional allies of South Korea and Japan” all had already concluded that the test violated U.N. resolutions.

“We will work with our friends and our allies to stand up to this behavior, and we will redouble our efforts toward a more robust international nonproliferation regime that all countries have responsibilities to meet,” Obama said.

After the 2006 test, the United States and other countries agreed to give North Korea a million tons of fuel oil in exchange for dismantling its nuclear facilities. The country started to dismantle its facilities in 2007, but stopped in 2008 in a dispute over how to verify its progress.

Then, when it tried to test a missile on April 5 and was widely criticized, North Korea said it would restart its nuclear program.

Japanese Prime Minister Taro Aso said the U.N. Security Council would meet at 4:30 p.m. Monday.

“North Korea’s nuclear test poses a grave challenge to nuclear nonproliferation and clearly violates U.N. Security Council resolutions,” he said in Tokyo. “We are not tolerating this at all.”

The new North Korean test triggered calls for Obama to press China and Russia – which have veto power as permanent U.N. Security Council members – to end their resistance to tough U.N. sanctions on North Korea.

However, a diplomat in Vienna, the headquarters of the U.N. International Atomic Energy Agency, said that earlier sanctions failed to persuade Pyongyang to abandon its nuclear weapons and ballistic missile programs. The diplomat agreed to speak only on the condition of anonymity because he wasn’t authorized to comment on the repercussions of the North Korean test.

“Everyone knows that tighter sanctions didn’t work in the past and won’t work now,” said the diplomat, who closely follows the North Korea program. “Sanctions haven’t worked. Security Council resolutions haven’t worked. This means there is a need for (new) diplomacy.”

The diplomat also said that the United States and other countries appear to have “overestimated” Beijing’s ability to use the considerable food and economic aid it provides Pyongyang to influence North Korean behavior.

He said the Obama administration should explore a fresh diplomatic approach, noting that it’s “shown the wherewithal to think outside the box” by offering to hold direct talks with Iran on the dispute over its nuclear program.

“The complicating factors are the views of South Korea and Japan,” he continued. “They have their own issues with the DPRK (Democratic People’s Republic of Korea).”

The diplomat said that the test appeared to be aimed at pressing North Korea’s demand for direct talks with the United States on a treaty to end the 1950-53 Korean War, something that successive U.S. administrations have refused to do without the participation of South Korea. A July 27, 1953, armistice agreement signed by the North Korean, Chinese and U.N. military commanders is still in place, but then South Korean President Syngman Rhee refused to sign it, and no peace treaty has ever been negotiated.

Some experts said the North Koreans also want the United States to recognize them as a nuclear weapons power, something that Washington has refused to do because it would legitimize Pyongyang’s undermining the global system to curb the spread of nuclear weapons.

“They want to be treated as a legitimate nuclear power, and there is no way we can do that,” said Daniel Sneider of Stanford University’s Walter H. Shorenstein Asia-Pacific Research Center. “In their minds, it is compensation for weakness. It is a weak state, a collapsing state.”

Such recognition, however, would undermine U.S. security guarantees to South Korea and Japan, which could then consider developing their own nuclear arsenals in response, Sneider said.

Pyongyang, the diplomat in Vienna said, also may be reinforcing its decision to renounce the 2005 so-called Six-Party agreement with China, Japan, Russia, South Korea and the U.S. to abandon its nuclear arms program. “These bangs are for political mileage, not war fighting,” he said.

Pyongyang renounced the 2005 accord – and 2007 agreements on steps to implement it – after the U.N. Security Council on April 13 condemned its test launch of a three-stage long-range missile over the Sea of Japan, and imposed sanctions on three North Korean firms. Pyongyang also expelled U.N. and U.S. nuclear inspectors and said it was restarting its plutonium production reactor at Yongbyon.

The United States and Japan extended their own unilateral sanctions on North Korea for the missile launch.

The diplomat said the nuclear test likely will prompt experts to “revisit” their estimates of the amount of plutonium the North Koreans have extracted from the Yongbyon reactor.

Current estimates put the North Korean nuclear arsenal at 6-12 warheads, and many experts had said after its partially successful 2006 underground blast that it couldn’t afford to expend its limited stockpile on any more tests.

The latest test comes at a critical time.

North Korea dictator Kim Jong-il, 67, is in poor health, believed to have suffered a stroke last August. His health could be driving him to seek nuclear capability before dying or handing over power. His succession, like most everything about the isolated communist dictatorship, isn’t clear.

Sneider said that the nuclear and missile tests and the renunciation of the six-party accord appear to show that a commission of senior military officers and civilians involved in military industries and the missile and nuclear weapons programs is overseeing regime policy following Kim’s illness.

A leading member of the commission is Kim’s brother in law, Jang Song Paek, who may be acting a “regent” until the youngest of Kim’s three sons, Jong-Un, 25, whom he’s apparently tapped as his successor, is old enough to step into his father’s shoes, Sneider said.

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