Tokyo: Japan's defence ministry today deployed three planes to collect possible radioactive material following North Korea's claimed hydrogen bomb test, officials said.
"To understand the impact of possible radioactive materials released by the test, Air Self-Defence Force planes have collected dust in the air," Chief Cabinet Secretary Yoshihide Suga told a press conference.
"It is currently being sent to the Japan Chemical Analysis Centre," he said, adding that the T4 training planes stayed in Japanese airspace.
Today's nuclear test was the North's fourth after previous ones in 2006, 2009 and 2013, though many experts cast doubt on Pyongyang's claim that it was a successful test of a hydrogen bomb.
They said the seismic activity suggested a less powerful atomic device.
In Vienna, the Comprehensive Test Ban Treaty Organisation said the magnitude was very similar to 2013.
But this did not rule out a H-bomb, since the nature of the device could not be determined from seismic data alone.
The organisation said further clues might come from analysis of radionuclides (radioactive particles). But this could take anything from several days to several months, and they might not be detected anyway.
Randy Bell, director of its international data division, said the data today was very consistent with the 2013 test and the seismic magnitude of 4.9 was the same.
"But to try to ascertain the very particular nature, such as whether this was nuclear or non-nuclear, or which type of nuclear, is not appropriate at this time," he told reporters.
In Tokyo Suga said no abnormal levels of radiation had been detected through monitoring posts across Japan as of early Wednesday evening.
The results collected by the planes are expected to be released on Thursday, an official with Japan's Nuclear Regulation Authority told reporters.
Last month North Korean leader Kim Jong-Un suggested Pyongyang had already developed a hydrogen bomb, though the claim was questioned by international experts.
