Responding to Japanese media reports that North Korea had carried out a second test, Chi Heon-cheol, director of the South Korea's Earthquake Research Centre, said no activity has been recorded suggesting an explosion.

"There's no signal from North Korea, even no small event," he told The Associated Press.

"There has been no activity in the last two hours," US Geological Survey official Rafael Abreu said.

The agency can detect most tremors if they are above 3.5 magnitude, he said.

In Tokyo, a Japanese foreign ministry official said earlier on Wednesday that the Japanese government suspects that North Korea conducted a second nuclear test. The official, speaking on condition of anonymity, declined to give details.

Japanese broadcaster NHK reported that North Korea may have conducted a second test.

Citing Japanese government seismic sources, NHK said: "North Korea appears to have tested a second nuclear device following Monday's reported test."

NHK said that it had received information from the source that there was a tremor in North Korea on Wednesday and checks were now underway on the possibility of a nuclear test.

Denial

Shinzo Abe, the Japanes prime minister, told a parliamentary panel on Wednesday that Japan does not have information indicating that North Korea had conducted a second nuclear test.

Taro Aso, the Japanes foreign minister, told the same panel that Japan had gained information that the reclusive communist state might conduct a second test on Wednesday but that it had not been confirmed.

The Chinese foreign ministry said that he was unaware as to whether North Korea might have conducted a second nuclear test.

In Washington, the White House's National Security Council said it has "no confirmation" of a possible second nuclear test.