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* North Korea carried out nuclear tests in 2006, 2009
* Says it is ready for more after U.N. censure move
* Agency has more than 270 monitoring stations worldwide
VIENNA, Jan 30 (Reuters) - The international agency that monitors nuclear tests said on Wednesday it would have no trouble detecting the seismological activity around any nuclear test by North Korea "within minutes" but it could take at least several days to confirm whether it was an atomic explosion.
North Korea, which detonated nuclear explosions in 2006 and 2009, vowed last week to conduct more rocket and nuclear tests after the U.N. Security Council censured it for a December long-range missile launch.
South Korea and others who have been closely observing activities at the North's known nuclear test grounds believe Pyongyang is technically ready to go and is awaiting the final word from supreme leader Kim Jong-un.
"The information from seismic stations reaches us within minutes," said Annika Thunborg, a spokeswoman for the preparatory commission of the Comprehensive Nuclear-Test Ban Treaty Organization (CTBTO). "We are confident that we will detect such a test."
In North Korea's previous tests, "we were able to say within a couple of hours that it was an explosion, that it was not an earthquake," Thunborg said. The data then had to be studied further to analyse whether it was an atomic blast.
Although estimates vary widely, North Korea is believed by some experts to have enough fissile material for about a dozen plutonium warheads. Intelligence reports suggest that it has been enriching uranium to supplement that stock and give it a second path to the bomb.
The test-ban treaty has actually not yet taken into effect because not all holders of nuclear technology have signed up for it. But the preparatory organisation already monitors possible breaches, deploying more than 270 stations worldwide to look out for signs of atomic tests, including seismic waves and radioactive traces.
The "seismic system is able to distinguish between a natural event, an earthquake, and a man-made event, an explosion, so we are confident in terms of the reliability of the system," Thunborg said.
Although no radioactivity was discovered from the North's test four years ago, Thunborg said it was possible to say with "reasonable certainty" that it was a nuclear test because of other data, including the size and depth of the explosion.
In 2006, she said, "you had smoking gun evidence, you detected radioactivity which clearly came from North Korea."
Nuclear expert James Acton at the Carnegie Endowment for International Peace think tank said seismic monitoring was "a reliable means of detecting nuclear tests and estimating their yields."
Seen as a cornerstone of efforts to free the world of atomic bombs and negotiated in the 1990s, the CTBT enjoys wide support around the world. But of the five officially recognised nuclear weapon states, the United States and China still have to ratify it. (Reporting by Fredrik Dahl; Editing by Sonya Hepinstall)