By Li Hongmei

Possible radioactive traces from February's nuclear test by the Democratic People's Republic of Korea (DPRK) have been detected by a monitoring station in Japan for the first time, although it remains unclear what fissile material Pyongyang used, monitoring organization CTBTO said 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 Organization (CTBTO) said.

"This coincides very well with announced nuclear test by the DPRK that occurred on 12 February 2013, 55 days before the measurement."

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

The DPRK 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 Pyongyang.

It would also raise international concerns that the DPRK might pass on weapons-grade uranium, or the necessary technology and knowhow to make it.

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 that the source was Japan's crippled Fukushima nuclear plant.

The detection was made at Takasaki, Gunma Prefecture, 1,000 kilometers from the DPRK's test site. Lower levels were picked up at Ussuriysk, Russia, one of several hundred sites worldwide reporting to the CTBTO.