VIENNA (AFP) - It is likely too late to determine what kind of fissile material North Korea used in its latest nuclear test, since no radioactive traces have been detected, an international monitoring group said on Wednesday.

"It is very unlikely that we will register anything... at this late stage," said Ms Annika Thunborg, a spokesman for the Vienna-based Comprehensive Test Ban Treaty Organisation (CTBTO).

The body collects data from monitoring stations around the world detecting seismic activity and monitoring the atmosphere for radioactive particles or noble gases that may escape from an underground nuclear explosion.

The detection of so-called radionuclides can take several weeks. A station in Canada detected the radioactive noble gas xenon 133 two weeks after North Korea's first nuclear test in 2006, for example, but none was detected after its 2009 explosion.

North Korea used plutonium in 2006 and 2009 and any discovery that it used highly enriched uranium for its third test on February 12 would mark a significant technological step for the impoverished and unpredictable regime in Pyongyang.

It would also raise international concerns that North Korea might pass on weapons-grade uranium, or the necessary technology and knowhow to make it, to other "rogue states" or "terrorists" seeking to make crude nuclear explosive devices.

The North has substantial deposits of uranium ore and it is much easier secretly to enrich uranium in centrifuges rather than producing plutonium in a nuclear reactor.

Weapons-grade uranium is also significantly easier and safer to smuggle than plutonium.