South Korean experts have not detected any radioactive isotopes from North Korea’s nuclear test, hampering efforts to assess the device.
Eight samples had been analyzed but nothing found, the Nuclear Safety and Security Commission said.
Finding certain isotopes – xenon gases in particular – would help experts determine whether a plutonium or uranium-based device was used.
But a well-contained test could yield no radioactive isotopes, experts say.
South Korean planes and ships were sent out immediately after the test to collect samples, as was a Japanese plane.
“Two days since the North’s nuclear test, the commission has completed analyzing eight samples, but no radioactive isotopes have been discovered as of 15:00 Thursday,” the commission said.
No changes had been recorded at 122 unmanned radiation monitoring systems across the country, it added.
North Korea said on Tuesday that it had carried out an underground nuclear test, which it claimed involved a miniaturized but more powerful device.
Seismic activity triggered by the test at the Punggye-ri site in the north-east of the country was detected by several nations.
The UN Security Council has condemned the test – North Korea’s third, following tests in 2006 and 2009 – and promised action.
Xenon isotopes were detected after the first test but not after the second. Experts say finding it can be a matter of luck and has to be done quickly because it decays rapidly.
North Korea’s first two tests involved plutonium, but it is believed to have a uranium-enrichment programme.
While it has depleted its stocks of “reactor-grade” plutonium needed to make the weapons-grade variety, the country has plentiful reserves of uranium ore.
South Korean experts have not detected any radioactive isotopes from North Korea’s nuclear test, hampering efforts to assess the device
North Korea’s neighbors and the US, meanwhile, are working to coordinate a response to the test.
On Thursday South Korea’s military conducted exercises on both coasts, Yonhap news agency said, while the defence ministry unveiled a missile that it said could hit “precise targets” like “windows of a North Korean command office”.
The National Assembly also adopted a resolution condemning the nuclear test. The two Koreas are still technically at war following the armistice of 1953 that ended armed conflict on the Korean peninsula.
Amid a flurry of diplomatic activity over the issue, US President Barack Obama spoke on the phone with Japanese Prime Minister Shinzo Abe for about 20 minutes, according to Kyodo news agency.
“They pledged to work closely together to seek significant action at the United Nations Security Council and to co-operate on measures aimed at impeding North Korea’s nuclear and ballistic missile programmes,” the White House said in a statement.
The Australian government, meanwhile, has also decided to postpone a visit by North Korean diplomats exploring the possibility of reopening an embassy in the capital, Canberra, this weekend, because of the test.
“We postponed the arrival of North Korean diplomats in Australia as a gesture following this detonation,” Australian Foreign Minister Bob Carr said.
Bob Carr, however, added that there are still plans to re-establish the embassy, saying that “there’s value in having a North Korean diplomatic presence here”.
North Korea closed its embassy there in 2008 due to financial issues.
North Korea confirms it has successfully carried out its third underground nuclear test, a move that has drawn international condemnation.
Pyongyang said the test involved a “miniaturized” device and was carried out in a “perfect manner”.
The confirmation came three hours after seismic activity was detected at North Korea’s nuclear test site.
President Barack Obama called for “swift” and “credible” international action in response.
He said the “provocative” nuclear test did not make North Korea more secure, adding that Washington would remain vigilant and steadfast in its defence commitments to its allies in Asia.
The United Nations had warned of “significant consequences” if Pyongyang went ahead.
UN Secretary General Ban Ki-moon has condemned the test as a “clear and grave violation” of UN resolutions and a “deeply destabilizing” provocation.
The Security Council is to hold an emergency meeting at 14:00 GMT on Tuesday in New York, diplomats say.
North Korea previously conducted nuclear tests in 2006 and 2009. It announced in January that it would conduct a third as a response to UN sanctions that were expanded after its December rocket launch.
Confirmation of the test came in a statement from state-run KCNA news agency.
“It was confirmed that the nuclear test that was carried out at a high level in a safe and perfect manner using a miniaturized and lighter nuclear device with greater explosive force than previously did not pose any negative impact on the surrounding ecological environment,” KCNA said.
North Korea confirms it has successfully carried out its third underground nuclear test, a move that has drawn international condemnation
The claim to have tested a “miniaturized” device is likely to alarm observers. The US and North Korea’s neighbors fear Pyongyang’s ultimate goal is to produce a nuclear device small enough to fit on a long-range missile, something it is not yet believed to have mastered.
In December it put a satellite into space using a three-stage rocket – a move condemned by the UN as a banned test of missile technology.
North Korea said the nuclear test – which comes on the eve of President Barack Obama’s State of the Union address – was to “to protect our national security and sovereignty against the reckless hostility of the United States”.
It is the first such test under new leader Kim Jong-un, who took over the leadership after his father Kim Jong-il died in December 2011.
Activity had been observed at the Punggye-ri nuclear test site for several months.
Seismic activity was then detected by monitoring agencies from several nations at 11:57 a.m. A shallow earthquake with a magnitude of 4.9 was recorded, the US Geological Survey said.
Both South Korea and Japan convened emergency meetings of their national security teams shortly afterwards.
“This is an unacceptable threat to the security of the Korean peninsula and north-east Asia, and a challenge to the whole international community,” South Korea’s presidential national security adviser Chun Young-woo said.
Japanese Prime Minister Shinzo Abe said his government would “consider every possible way to address this issue”.
The US, South Korea and Japan had all warned Pyongyang not to go ahead with the test. China, North Korea’s closest ally and biggest trading partner, had also called for restraint.