Japan said Thursday it has detected no changes in radiation levels within its territory, following North Korea's claimed test of a hydrogen bomb the day before.

Concern in Japan over potential radiation drifting across the sea from North Korea skyrockets whenever it conducts underground nuclear tests, though none has ever been traced to the country after its three previous ones.

Japan is particularly sensitive to North Korea's nuclear and missile tests as prevailing winds blow from the Korean peninsula towards Japan and Pyongyang's ballistic rockets have flown over Japan into the Pacific Ocean.

"There was no particular change" so far in levels of radiation after Pyongyang's surprise underground explosion, Japan's Nuclear Regulation Authority (NRA) said in a statement.

It added that "no artificial radioactive nuclides" were detected from air samples collected by three Japanese air force planes which flew over the archipelago on Wednesday.

Also, some 300 monitoring posts across the country registered no meaningful change in radiation levels between Wednesday afternoon and Thursday morning as of 8:00 am (2300 GMT Wednesday), the NRA said.

Satoshi Yamamoto, an NRA official, told reporters late Wednesday that in general radioactive material is not even expected to be released in the case of underground nuclear tests.

Nevertheless, nuclear authorities are strengthening their monitoring activities "to confirm that radiation levels remain normal" after North Korea's test, Yamamoto said.

Japan did not detect artificial radioactive materials at the time of Pyongyang's three previous nuclear explosions between 2006 and 2013, he said.

Fresh results from a second round of flight by air force planes Thursday will be announced Friday, the NRA said.

But even if artificial radioactive material is detected, it would be difficult to discern whether it came from atomic bombs or a more destructive hydrogen device as the only material an H-bomb produces that an atomic bomb does not is helium, Yamamoto said.

Separately, Japanese Foreign Minister Fumio Kishida and his South Korean opposite number Yun Byung-Se agreed Thursday in a telephone call that their countries will "closely cooperate" with other key players including China and the United States in responding to the test, Japan's foreign ministry said in a statement.