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North Korea may have conducted a second nuclear test, Japanese public broadcaster NHK reported, but South Korean officials quickly denied the report.
In Washington, a spokesman for the National Security Council said the United States had “no confirmation” of a possible second nuclear test by Pyongyang.
Quoting unnamed government officials, NHK said Japan had detected a tremor from the neighboring communist country early Wednesday which may indicate a second nuclear test.
Top Japanese officials could not confirm the report.
“I have not heard any information with that kind of sign,” Prime Minister Shinzo Abe said in parliament when asked about the report.
Foreign Minister Taro Aso, however, did not rule out the possibility of a second nuclear test.
“I am aware of information that North Korea may possibly have carried out a second nuclear test today, but there is no confirmation of it,” Aso said.
The Japan Meteorological Agency said it was still assessing whether there had been any seismic waves.
In Seoul, the South Korean presidential office quickly denied the Japanese reports.
“So far we have not received reports of fresh seismic activity,” a presidential spokesman told AFP in the South Korean capital.
A South Korean foreign ministry official, quoted by Yonhap news agency, said there have been no signs of the North moving to conduct a second test.
“We have no information about it yet,” the official said, speaking on condition of anonymity.
Chi Heon-Cheol, head of the Korea Earthquake Research Center, told SBS television that no seismic activity has been detected.
“No signs of any explosion were detected in North Korea this morning. However, there was an small-scale earthquake in the East Sea (Sea of Japan),” he said.
Pyongyang announced Monday it had carried out its first-ever nuclear test, triggering global condemnation.
North Korea, which declared last year it had nuclear weapons but until this week had not purported to prove it, is believed by experts to have at most only a small number of atomic bombs.
Japan, the United States and South Korea were still trying to verify that Monday's test was genuine.
On Monday, Japan detected seismic waves measuring 4.9 on the Richter scale Monday in North Korea but it was still looking at atmospheric particles to see if it was a nuclear rather than a conventional blast.
A US official, speaking to AFP on condition of anonymity in Washington, said Washington assumes that the North Korean test Monday was a nuclear bomb, based on the seismographic data.
“The working assumption certainly is that it was a nuclear explosion,” the official said.
“The precise details of what happened — that's the challenge to the analysts in the community right now,” he said. “The hard conclusions in terms of precisely what happened have not been reached.”
But officials said seismographic data gathered by the United States pointed to an explosion with a yield of as low as 200 tonnes.
First-time nuclear tests are typically in the several kilotonne range, according to US officials.
