North Korea carries out 'biggest ever' nuclear test: South Korea

North Korea is believed to have conducted a fifth nuclear test, its most powerful to date, South Korea's military said today 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."

North Korea has been hit by five sets of United Nations sanctions since it first tested a nuclear device in 2006. Since the January test concern has also grown over a series of provocative ballistic missile launches.

Pyongyang test-fired three missiles Monday as world powers gathered for a G20 meeting in China, with leader Kim Jong-Un hailing the tests as "perfect", and US President Barack Obama warning it would only up the pressure.

North Korea has always insisted it will continue nuclear tests despite global condemnation and toughened sanctions. Experts say they are likely aimed at refining warhead design and reliability as well as increasing yield.Outside monitors will now attempt to analyse the yield -- the strength of the bomb -- to try to determine what kind of a breakthrough it represents.

North Korean nuclear tests are usually heralded by chatter among analysts about preparations at Punggye-ri but there had been little discussion in recent weeks over any signs of a test.