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SEOUL, South Korea (AP) -- North Korea on Friday conducted its fifth atomic test, producing its biggest-ever explosive yield, South Korean officials said after monitors detected unusual seismic activity near the North's northeastern nuclear test site.
South Korea's Joint Chiefs of Staff said in a statement that "artificial seismic waves" from a quake measuring 5.0 were detected near the Punggye-ri test site.
The South's Defense Ministry said it believed the North conducted a nuclear test, while European and U.S. monitoring services also detected similar seismic activity, with the U.S. Geological Survey calling it an "explosion" on its website.
A second North Korean nuclear test this year would raise serious worries in Washington because the North's nuclear tests are part of a push for a nuclear-armed missile that could one day reach the U.S. mainland. A second nuclear test this year would be a defiant response to Western pressure on Pyongyang to halt its nuclear ambitions. The country has previously conducted tests every three to four years.
A South Korean Defense Ministry official, who refused to be named because of office rules, said that Seoul detected an estimated explosive yield of 10 kilotons and assessed that it was from a nuclear test.
After the fourth test, in January, South Korean lawmaker Lee Cheol Woo said Seoul's National Intelligence Service told him that an estimated explosive yield of six kilotons was detected.
The 5.0 magnitude earthquake 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.