North Korea said Friday it conducted a “higher level” nuclear warhead test explosion, which it trumpeted as finally allowing it to 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.

SEOUL, South Korea — 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 that it showed the “fanatic recklessness of the Kim Jong Un government.”