SEOUL, South Korea (AP) -- North Korea said it conducted a "higher level" nuclear test explosion on Friday that will allow it to finally build "at will" an array of stronger, smaller and lighter nuclear weapons. It was the North's fifth atomic test and the second in eight months.
South Korea's president called the detonation, which Seoul estimated was the North's biggest-ever in explosive yield, an act of "fanatic recklessness." Japan called North Korea an "outlaw nation."
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.
Seoul vowed to boost psychological warfare efforts by increasing the number of propaganda loudspeakers along the rivals' border, the world's most heavily armed, and the number of hours of anti-North Korean broadcasts.
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 center 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." Kim is the North Korean leader.
Park's office said she spoke with U.S. President Barack Obama about the test by phone from Laos, where she attended a regional summit. Park said South Korea will employ all available measures to put more pressure on North Korea, which had previously conducted nuclear tests every three to four years.
Japanese Chief Cabinet Secretary Yoshihide Suga called North Korea an "outlaw nation in the neighborhood" and said Japan will consider stepping up its sanctions against the North.
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 percent 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.
Chinese state media said the nation's environmental protection agency started nuclear radiation monitoring. Japanese planes began to collect air samples from national air space to analyze possible radioactive materials. Tokyo Gov. Yuriko Koike said Japan's capital city is also testing water samples and monitoring radiation levels in the air.
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 Defense 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.
The test will lead to a strong push for new, tougher sanctions at the United Nations and further worsen already abysmal relations between North Korea and its neighbors. The North is already one of the most heavily sanctioned places on Earth, and many question whether the penalties work.
China opposed North Korea's nuclear test, a key denunciation by the North's economic lifeline and only major ally.
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.