North Korea has said it has carried out a "higher level" nuclear warhead test explosion which will allow it to finally build "at will" an array of stronger, smaller and lighter nuclear weapons.
It is Pyongyang's fifth atomic test and the second in eight months.
South Korea's president, Park Geun-hye, called the detonation - which Seoul estimated as the North's biggest-ever explosive yield - an act of "fanatic recklessness".
The North's boast of a technologically game-changing nuclear test defies both tough international sanctions and long-standing diplomatic pressure to curb its nuclear ambitions.
Hours after Seoul noted unusual seismic activity near the North's north-eastern nuclear test site, Pyongyang 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 standardised to be able to be mounted on strategic ballistic rockets".
"The standardisation 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," the North said.
"This has definitely put on a higher level (the North's) technology of mounting nuclear warheads on ballistic rockets."
South Korea's main spy agency told politicians in a closed-door briefing after the test that it does not think Pyongyang currently has the ability to develop miniaturised nuclear weapons which can be mounted on ballistic missiles, but intelligence officials expressed worries that the North's efforts to do so are progressing quicker than previously thought, said Kim Byungkee, of the opposition Minjoo Party.
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% 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 there was no radioactive material leaked, but the explosion put the region on edge.
Chinese state media reported that the nation's environmental protection agency started nuclear radiation monitoring.
Japanese planes began to collect air samples from national air space to analyse possible radioactive materials.
Tokyo Governor Yuriko Koike said Japan's capital city is also testing water samples and monitoring radiation levels in the air.