SEOUL - Urgent efforts to find out the type of device detonated in North Korea's latest nuclear test appeared to be getting nowhere Thursday, with South Korean experts unable to detect any radioactive fallout.
The North's test on Tuesday triggered an immediate scramble to collect and analyse any fallout data that might provide crucial clues about the nature of the test and the progress Pyongyang's nuclear weapons programme has made.
While seismic data was able to shed light on the likely yield of the underground test - estimated at 6-7 kilotons - the main hunt was for elusive radioisotopes that might confirm the type of fissile material that was used.
Experts are particularly keen to establish whether the North switched from plutonium - used in the 2006 and 2009 tests - to a new and self-sustaining nuclear weaponisation programme using highly enriched uranium.
The South's state-run Nuclear Safety and Security Commission said Thursday it had analysed eight atmospheric samples apparently collected by warships and air force planes equipped with highly sensitive detection devices.
"No radioactive isotope has been found yet," the commission said in a statement.
Their priority target was traces of xenon gases released in the detonation that would point to the weapon type. "We are analysing samples and xenon has not been found," the commission statement said.
If the underground test was well contained, it is quite possible there would be little or no radioactive seepage into the atmosphere.
And even if some gases did escape, scientists stress there is a large amount of luck involved in collecting them. No xenon gases were detected after the North's 2009 test.
As well as the military detectors, the commission said there were 122 automated devices across South Korea that were continually capturing and analysing air samples.
The detection effort is running on a very tight deadline. Xenon-133m, a metastable isotope needed to pin down the fissile material type, has a half-life of just over two days.
Proof of a uranium test would confirm what has long been suspected: that the North can produce weapons-grade uranium, doubling its pathways to building more bombs in the future.
The North has substantial deposits of uranium ore and it is much easier secretly to enrich uranium in centrifuges rather than enriching plutonium in a nuclear reactor.
In an apparent effort to showcase its own military muscle on Thursday, South Korea's Defence Ministry provided a video demonstration of a newly deployed cruise missile capable of striking precision targets in the North.
"With this missile, we could hit any facility, equipment or individual target in the North anywhere, at any time of our choosing," Major General Ryu Young-Jeo told a special press briefing.
Defence Ministry spokesman Kim Min-Seok said the missile was accurate enough to target "the office window of the North's command headquarters."
It has "deadly destructive power" that could "restrain the enemy headquarters' activities" during wartime, Kim told reporters.
The day after the nuclear test, South Korea said it would speed up the development of longer-range ballistic missiles that could also cover the whole of North Korea.
Last October South Korea reached a deal with the United States to almost triple the range of its ballistic missile systems - with Seoul arguing it needed an upgrade to counter the North's own missile development.
The United States has 28,500 troops in South Korea and guarantees a nuclear "umbrella" in case of any atomic attack. In return, Seoul accepts limits on its ballistic missile capabilities.