North Korea conducted its third underground nuclear test on Tuesday, underscoring a disregard for an international community that has already isolated the totalitarian state from the global economy.
The underground test "of a smaller and light A-bomb" was successful, the official Korean Central News Agency said in a statement.
South Korea measured an artificial 4.9 magnitude earthquake at the North's Punggye-ri testing site at 11.57am (Korea time), and its Defence Ministry estimated the yield at six to seven kilotonnes, bigger than the previous two tests.
The atomic bomb dropped on Hiroshima had a yield of about 15 kilotonnes.
A detonation three weeks after the United Nations tightened its sanctions indicates Kim Jong Un backs the military-first policy of his late father as he solidifies hold of the country he took charge of 14 months ago.
The incident comes amid political transitions in Asia and the United States, posing an early challenge to policy makers from President-elect Park Geun Hye in South Korea to US Secretary of State John Kerry.
"It just shows the failure of diplomacy and the continuation of the old broken-record story, the game of cat- and-mouse where North Korea is trying to intimidate its neighbours and the neighbours are trying to push for more deterrents and an escalation of tensions is inevitable," said Leonid Petrov, a North Korean analyst and associate researcher at the Australian National University in Canberra.
Yield Differential
Visitors look towards North Korea from a South Korean observation post in Goseong, 170 km northeast of Seoul, near the Demilitarised Zone dividing the two Koreas on Tuesday. (AFP photo)
North Korea previously conducted two tests at its site in North Hamgyong province, both with plutonium devices. The first in October 2006 yielded less than one kilotonne and the second in May 2009 between five to six kilotonnes, South Korean Defense Ministry spokesman Kim Min Seok said in a televised briefing. He said it will be difficult to determine what fissile material -- plutonium or highly-enriched uranium -- the North used.
"The point is whether the North has successfully miniaturised and lightened the warhead," said Jeung Young Tae, senior research fellow at the Korea Institute for National Unification in Seoul. "The threat is in whether North Korea can deliver a nuclear warhead, regardless of how powerful it is."
North Korea has enough plutonium to produce between six and 18 nuclear weapons, according to the Washington-based Institute for Science and International Security.
US President Barack Obama called the test a threat to regional stability that warranted "swift and credible action" from the international community.
US Threat
"North Korea's nuclear weapons and ballistic missile programs constitute a threat to US national security and to international peace and security," Obama said in a statement from Washington, adding that his administration will "continue to take steps necessary to defend ourselves and our allies."
South Korea's currency and benchmark Kospi index of shares initially dipped after the news of an artificial earthquake being detected in the north. The won gained 0.5% to 1090.69 per dollar and the Kospi closed 0.3% lower at 1,945.79 in Seoul. The MSCI Asia Pacific Index retained its gains, with markets including Hong Kong and Singapore off for lunar new year celebrations.
The South's military will accelerate expanding its capabilities by deploying at an early stage its extended-range missiles, chief national security adviser Chun Yung Woo said in a live television briefing. It also remains on alert for the possibility of additional provocations, Chun said.
UN Meeting
Park, who takes office Feb 25, strongly condemned the launch, vowing that her new government will not allow a nuclear- armed North Korea, her spokeswoman Cho Yoon Sun said.
The UN Security Council in New York will convene an emergency session to discuss a response against Tuesday's atomic experiment, South Korean Foreign Ministry spokesman Cho Tai Young told reporters in Seoul.
Kim Jong Un last month vowed "high-profile" retaliation against the US and its allied for tightening UN sanctions after his country successfully launched a long-range rocket. His government threatened to bolster its nuclear capability in protest against the US-led effort that was supported by China, Pyongyang's most powerful ally.
North Korea notified the US and China on Monday that it would conduct a nuclear test, the Defense Ministry spokesman said. Japan was informed by the US, Chief Cabinet Secretary Yoshihide Suga said.
Tuesday's atomic detonation comes exactly two months after the North successfully test-fired a long-range rocket. South Korea's military estimates the missile's range to be more than 10,000 kilometres.
Security Challenges
It also adds to existing security challenges in northeast Asia, where tensions over territorial disputes have escalated in the past year. Japan this month protested that a Chinese vessel had used weapons-targeting radar on one if its destroyers, a charge that China denied. Ties between the two nations have deteriorated over rival claims to islands in the East China Sea known as Senkaku in Japan and Diaoyu in China.
It is the second time Abe has to manage the implications of a North Korean nuclear test, as the first detonation came during his last time as prime minister. Abe, who took office in December, has proposed the first increase in Japan's defense budget in 11 years to cope with mounting intrusions by Chinese craft in Japanese-administered waters.
Abe in parliament called the test "extremely regrettable," after earlier meeting with his national security team. He told reporters Japan may seek tougher sanctions on North Korea "in consultations with the international community."
China's Influence
For China, the test underscores the limited influence that North Korea's closest ally has on the country's policies. China is forecast next month to anoint Xi Jinping as its new president after he took over the Communist Party in November. The country is in the midst of celebrating the arrival of the year of the snake.
China expressed "firm opposition" to the test and called on all parties to respond calmly, according to a statement on the Ministry of Foreign Affairs' website.
The incident also comes at a time of transition in the US, with Obama's new security team taking shape as his second administration gets under way. Kerry, the former Democratic senator from Massachusetts who succeeded Hillary Clinton as the top American diplomat, may visit South Korea next month, according to a Yonhap News report on Tuesday.
In April 2009, North Korea said it would restore its main nuclear reactor for making weapons-grade plutonium at Yongbyon, about 90 kilometres north of the capital Pyongyang. The regime denied having a separate uranium-enrichment program until September 2009, when it told the UN Security Council it was "weaponising" plutonium and had almost succeeded in highly enriching uranium.