SEOUL, Korea, Republic Of -- North Korea said it conducted a "higher level" nuclear test explosion on Friday that will allow it to finally build an array of stronger, smaller and lighter nuclear weapons, a move strongly condemned by the U.N. Security Council which promised new measures against Pyongyang.
The North's fifth atomic test and the second in eight months brought the U.N.'s most powerful body into emergency session, just three days after it strongly condemned North Korea's latest ballistic missile launches.
South Korea's president said the detonation, which Seoul estimated was the North's biggest-ever in explosive yield, was an act of "fanatic recklessness" and a sign that leader Kim Jong Un "is spiraling out of control." President Barack Obama condemned the test and said the U.S. would never accept the country as a nuclear power.
North Korea's boast of a technologically game-changing nuclear test defied both tough international sanctions and long-standing diplomatic pressure to curb its nuclear ambitions. It will raise serious worries in many world capitals that North Korea has moved another step closer to its goal of a nuclear-armed missile that could one day strike the U.S. mainland.
The press statement agreed upon by all 15 Security Council members late Friday said diplomats will draft a new resolution in response to its earlier promise to take "further significant measures," if the North continued to defy the international community.
"In line with this commitment and the gravity of this violation, the members of the Security Council will begin to work immediately on appropriate measures" in a new U.N. resolution, the statement said. The measures will be under Article 41 of the U.N. Charter, which specifies non-military actions including sanctions, it said.
U.S. Ambassador Samantha Power said the council must use "every tool at its disposal" including new sanctions "to demonstrate to North Korea that there are consequences for its unlawful and dangerous actions."
"This is more than brazen defiance," Power told reporters at U.N. headquarters. "North Korea is seeking to perfect its nuclear weapons and their delivery vehicles so they can hold the region and the world hostage under threat of nuclear strikes."
What measures are included in a new resolution will largely depend on China, the North's major ally and neighbour which fears any instability on the Korean peninsula.
"All sides should refrain from mutual provocations and any actions that might be a threat to peace and security," China's U.N. Ambassador Liu Jieyi said after the meeting. "We believe it is more urgent than ever to work together to achieve denuclearization of the Korean peninsula (and) "to prevent proliferation and ... maintain peace and stability on the Korean peninsula."
In March, the Security Council adopted its toughest sanctions against North Korea in two decades in response to its nuclear test in January and a rocket launch. It took two months of negotiations mainly between the U.S. and China.
South Korea's U.N. Ambassador Oh Joon said he hopes agreement on a new resolution will come quickly.
Hours after South Korea noted unusual seismic activity near North Korea's northeastern nuclear test site, the North said in its state-run media that a test had "finally examined and confirmed the structure and specific features of movement of (a) nuclear warhead that has been standardized to be able to be mounted on strategic ballistic rockets."
"The standardization of the nuclear warhead will enable (North Korea) to produce at will and as many as it wants a variety of smaller, lighter and diversified nuclear warheads of higher strike power," North Korea said. "This has definitely put on a higher level (the North's) technology of mounting nuclear warheads on ballistic rockets."
North Korea, led by a third-generation dictatorship and wary of outsiders, protects its nuclear program as a closely guarded state secret, and the claims about advancements made in its testing could not be independently verified. But they centre on a technological mystery that has long bedeviled outside experts: How far has North Korea gotten in efforts to consistently shrink down nuclear warheads so they can fit on long-range missiles?
South Korea's main spy agency told lawmakers in a closed-door briefing after the test that it does not think North Korea currently has the ability to develop nuclear weapons that can be mounted on ballistic missiles, but intelligence officials expressed worries that the North's efforts to do so are progressing more quickly than previously thought, said Kim Byungkee, a lawmaker from the opposition Minjoo Party.
South Korean President Park Geun-hye strongly condemned the test, saying in a statement that it showed the "fanatic recklessness of the Kim Jong Un government as it clings to nuclear development."
She told a meeting of top security officials Friday night that, "We have to believe that Kim Jong Un's mental state is spiraling out of control because he is not listening to any words from the international community or neighbouring countries in his attempt to cling to power."
Obama condemned the nuclear test "in the strongest possible terms as a grave threat to regional security."
"The United States does not, and never will, accept North Korea as a nuclear state," he said in a statement. "Today's nuclear test, a flagrant violation of multiple U.N. Security Council resolutions, makes clear North Korea's disregard for international norms and standards for behaviour and demonstrates it has no interest in being a responsible member of the international community."
Japanese Prime Minister Shinzo Abe said he and South Korean President Park talked by telephone and agreed that North Korea's nuclear test and its recent missile launches show that it now poses a "different level of threat" requiring a new response.
South Korea's weather agency said the explosive yield of the North Korean blast would have been 10 to 12 kilotons, or 70 to 80 per cent of the force of the 15-kiloton atomic bomb the United States dropped on the Japanese city of Hiroshima in 1945. The North's fourth test was an estimated six kilotons.
North Korea said no radioactive material leaked, but the explosion put the region on edge.
In the North Korean capital, Pyongyang, residents were delighted.
"It's really great news," said Rim Jong Su, 42. "Now, I am full of confidence that if the enemies make any little provocations we will make a counter attack and we will surely win."
The 5.0 magnitude seismic event Friday is the largest of the four past quakes associated with North Korean nuclear tests, according to South Korea's weather agency. Artificial seismic waves measuring 3.9 were reported after North Korea's first nuclear test in 2006; 4.8 was reported from its fourth test this January.
North Korean leader Kim has overseen a robust increase in the number and kinds of missiles tested this year. Not only has the range of the weapons jumped significantly, but the country is working to perfect new platforms for launching them -- submarines and mobile launchers -- giving the North greater ability to threaten the tens of thousands of U.S. troops stationed throughout Asia.
North Korea has fired a total of 33 ballistic missiles since Kim took power in 2011, Seoul's Defence Ministry said. In comparison, North Korea fired 16 ballistic missiles during the 17-year rule of Kim's father, Kim Jong Il.
The seismic activity comes on the 68th anniversary of the founding of North Korea's government and just days after world leaders gathered in China for the Group of Twenty economic summit.
North Korea likely wanted to show the world that strong international sanctions following its fourth nuclear test and long-range rocket launch earlier this year haven't discouraged its efforts to advance its nuclear weapon and missile programs, according to Koh Yu-hwan, a North Korea expert at Seoul's Dongguk University.
North Korea's persistent pursuit of missiles and nuclear weapons has long been one of the most intractable foreign policy problems for U.S. administrations.
Diplomacy has so far failed. Six-nation negotiations on dismantling North Korea's nuclear program in exchange for aid were last held in late 2008 and fell apart in early 2009.
The Korean Peninsula remains technically at war, as the 1950-53 Korean War ended in an armistice, not a peace treaty.
Lederer reported from the United Nations. Associated Press writers Hyung-Jin Kim and Kim Tong-hyung in Seoul, Mari Yamaguchi in Tokyo, Robert Burns in Norway, Joseph Krauss in Cairo and Darlene Superville in Washington, D.C. contributed to this report.