VIENNA (AFP) - Possible radioactive traces from North Korea's February nuclear test have been detected for the first time, although it remains unclear what fissile material Pyongyang used, the monitoring organisation said on Tuesday.

"The ratio of the detected xenon isotopes (xenon-131m and xenon-133) is consistent with a nuclear fission event occurring more than 50 days before the detection," the Comprehensive Test Ban Treaty Organisation (CTBTO) said.

"This coincides very well with announced nuclear test by the DPRK (North Korea) that occurred on 12 Feb 2013, 55 days before the measurement."

It added, however, that the discovery by a monitoring station in Japan could not help it answer the key question of whether Pyongyang used plutonium or uranium in the blast.

North Korea used plutonium in its 2006 and 2009 tests and any discovery that it used highly enriched uranium for its third test 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 know-how to make it, to other "rogue states" or "terrorists" seeking to make crude nuclear explosive devices.

It is also possible that the so-called radionuclides were from a nuclear reactor or other atomic activity, and the CTBTO said it is currently examining the traces to see whether this is the case.

It ruled out, however, that the source was Japan's crippled Fukushima nuclear plant.

The detection was made at Takasaki, Japan, 1,000 kilometres from the North Korean test site. Lower levels were picked up at Ussuriysk, Russia, one of several hundred sites worldwide reporting to the CTBTO.