"The announcement is based on data collected around the clock," Lee Mun-Ki, head of the ministry's atomic energy bureau, was quoted by Yonhap news agency as saying.

Lee also said authorities have taken rainwater samples from places in the South where rain was reported after the test took place. The results of those tests will be released Friday.

If the samples showed a radiation level above that set by the International Atomic Energy Agency, the government would warn people to stay indoors, avoid eating outside and stay out of the rain.

Because the suspected blast site in North Hangyong province was about 250-300 kilometers (150-175 miles) from the nearest South Korean territory, the possibility of underwater reservoirs in the South being contaminated was remote, Lee said.

The official said Seoul was also looking into the possibility of radioactive particles seeping into the East Sea (Sea of Japan), and that water and seabed samples would be taken.

South Korea has leased a xenon nuclide detector from Sweden for specialist atmospheric tests.

But the state-run Korea Institute of Nuclear Safety said southwesterly breezes have been blowing from North Korea since Tuesday, making it very hard for detection devices to pick up any radiation.

A French nuclear official said Wednesday the test may never be confirmed because the blast was so weak -- although if the device was indeed nuclear, the test was likely a flop.

"There is a series of differentiations to be done" to sift out the blast from background noise among the seismic data recorded on Monday, said Xavier Clement, spokesman for France's Atomic Energy Commission.

"It is possible that this cannot be done, given the weakness of the signals compared to the background noise" of subterranean movement, he told AFP.

Clement agreed with other estimates that put the explosion at less than one kilotonne equivalent of TNT.

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