SEOUL, South Korea — 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. It was the North’s fifth atomic test and the second in eight months.

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.

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.”