A nuclear monitoring station in Japan has recorded what could be the first radioactive noble gases from February's nuclear test in North Korea.

The station in Takasaki, about 1,000km from North Korea's Punggye-ri nuclear test site, is operated by the Comprehensive Nuclear Test Ban Treaty Organization, which made the announcement yesterday.

Radioactive isotopes of the noble gas xenon – xenon-131m and xenon-133 – were detected, something the organisation called "rather unusual."

The measurements, almost two months after Pyongyang said it had carried out the underground detonation, gave no indication of whether plutonium or highly enriched uranium was used, the organisation said.

The time that had passed before the so-called noble gases were picked up made it "very difficult" to distinguish between the two fissile materials, said spokeswoman Annika Thunborg.

Noble gases are one of four things the organisation looks out for in its nuclear monitoring process, because the gases can be released by either slowly seeping through rock and sediment from underground to the surface after a nuclear test or come from activity at a test site.

The Comprehensive Nuclear Test Ban Treaty Organization said the ratio of the detected xenon isotopes is consistent with a nuclear fission event occurring more than 50 days before the detection, matching the timeframe for the North Korean nuclear test. Further, atmospheric modeling indicates the isotopes could be carried from Punggye-ri to Takasaki.

But the organisation isn't quite ready yet to say the detection is definitely linked to the February test in North Korea. "We are in the process of eliminating other possible sources that could explain the observations; the radionuclides could have come from a nuclear reactor or other nuclear activity under certain specific conditions, but so far we do not have information on such a release," Mika Nikkinen said in a statement.

The Comprehensive Nuclear Test Ban Treaty Organization's monitoring system was one of the first to indicat that North Korea had conducted its nuclear test. The organisation maintains almost 100 stations monitoring the tell-tale seismic activity that occurs with a nuclear test. On 12 February those stations recorded the event at 2:57:51 UTC (11:57:51 local time). The test was measured at magnitude 4.9 and located at latitude 41.313 degrees north and longitude 129.101 degrees east.