South Korea's seismic monitoring center said a magnitude 3.6 tremor felt at the time of the nuclear test wasn't a natural occurrence.
The head of South Korea's spy agency said the blast was equivalent to less than 1 kiloton of TNT, South Korea's Yonhap news agency reported. National Intelligence Service chief Kim Seung-kyu also told lawmakers that there were signs of suspicious movement at another suspected test site, Yonhap said.
The US Geological Survey said it recorded a seismic event with a preliminary magnitude of 4.2 in northeastern North Korea coinciding with the test claim, but survey official Bruce Presgrave said the agency was unable to tell if it was an atomic explosion or an earthquake.
Japanese Prime Minister Shinzo Abe said the test would mark the beginning of a "dangerous nuclear age" in north Asia.
Britain's Prime Minister Tony Blair said the test was a "completely irresponsible act."
"The breaking of a de facto global moratorium on nuclear explosive testing that has been in place for nearly a decade and the addition of a new state with nuclear weapon capacity is a clear setback to international commitments to move towards nuclear disarmament," the head of the International Atomic Energy Agency, Mohamed ElBaradei, said in a statement.
China yesterday said that Beijing "resolutely opposes" the North Korean nuclear test and hopes Pyongyang will return to disarmament talks.
South Korean President Roh Moo-hyun said the test would make it difficult for Seoul to maintain its engagement policy with its neighbor.
The South is reconsidering plans to ship 4,000 tons of cement of emergency relief to the North for floods it suffered in mid-July, a Unification Ministry official said on customary condition of anonymity.
Seoul cut off regular aid after the North conducted missile launches in July.
The North is believed to have enough radioactive material for about a half-dozen bombs.
It insists its nuclear program is necessary to deter a US invasion.
The North has active missile programs, but it isn't believed to have an atomic bomb design small and light enough to be mounted on a long-range rocket that could strike targets as far as the US
In Pyongyang, North Koreans went about their lives as usual on Monday, with no signs of heightened alert.
Red flags of the North's Korean Workers' Party draped buildings and lampposts to mark today's 61st anniversary of the party's founding.
The country's state TV read the report about the test during its regular newscasts. The item wasn't the top story and there were no images shown of the test.
RSS