By Jung Ha-Won
North Korea is believed to have conducted a fifth nuclear test, its most powerful to date, South Korea's military said Friday after monitors detected a 5.3-magnitude "artificial earthquake" near its main nuclear site.
A confirmed test by the isolated North would send tensions soaring over its nuclear and ballistic missile ambitions, which have already earned international condemnation and United Nations sanctions.
The quake was detected near North Korea's Punggye-ri nuclear test site as the country celebrates Foundation Day, which marks the anniversary of the founding of the nation in 1948.
"We believe that it was a nuclear test. We are trying to figure out whether it was successful. The blast measured about 10 kilotons," a defence ministry spokesman told reporters.
The North's third nuclear test, staged in February 2013, was previously considered the most powerful to date, with a yield of six to nine kilotons.
Authorities in Japan and South Korea said the tremor, measured at 5.3 by the US Geological Survey, showed every sign of another test, the latest since a January 6 test at Punggye-ri.
"The 10-kiloton blast was nearly twice the fourth nuclear test and slightly less than the Hiroshima bombing, which was measured about 15 kilotons," said Kim Nam-Wook from the South's meteorological agency said.
Japan's public broadcaster NHK reported the country's defence ministry was preparing to dispatch aircraft to analyse air samples to see if any radiation could be detected.
"If North Korea conducted a nuclear test, that can never be tolerated. We must lodge a strong protest," said Japanese Prime Minister Shinzo Abe.
The US National Security Council said it was aware of seismic activity in the region of the test site and was "monitoring and continuing to assess the situation in close coordination with our regional partners."
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