North Korea conducted a nuclear test after seismic activity measuring 4.9 magnitude was registered by the U.S. Geological Survey. Courtesy: CNN

POSSIBLE radioactive traces from a North Korean nuclear test in February have been detected for the first time, 1000km away in Japan.

The Comprehensive Test Ban Treaty Organization (CTBTO) said it had detected isotopes "consistent with a nuclear fission event", The Japan Times reports.

"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 CTBTO said.

"This coincides very well" with the North Korea’s announced nuclear test on February 12.

The detection at a monitoring station in Japan came 55 days after the explosion, The Japan Times reports.

The group said, however, that the discovery couldn’t 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.

It would also raise international concerns that North Korea might pass on weapons-grade uranium.

North Korea threatened nuclear attacks on the United States, South Korea and Japan after new U.N. sanctions were imposed in response to its latest atomic test. But U.S. officials have cast serious doubt on whether it could launch a nuclear missile.

Pyongyang's third nuclear test was registered virtually instantaneously via seismic signals around the world. But no radioactive traces that would have constituted conclusive proof were found in the immediate weeks afterwards.

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 the crippled Fukushima No.1 nuclear plant.

The detection was made in Takasaki, Gunma Prefecture, 1000 km from the North Korean test site. Lower levels were also picked up at Ussuriysk, Russia, one of several hundred sites worldwide reporting to the CTBTO.

The Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organisation (CTBTO) was set up in 1996 with its headquarters in Vienna, Austria. It is an interim organization tasked with building up the verification regime of the Comprehensive Nuclear-Test-Ban Treaty (CTBT) in preparation for the treaty's entry into force as well as promoting the treaty's universality.