South Korean experts say they have not detected any radioactive isotopes from North Korea's nuclear test, hampering efforts to assess the device, BBC News reported.

Eight samples had been analyzed but nothing found, the Nuclear Safety and Security Commission said.

Finding certain isotopes - xenon gases in particular - would help experts determine whether a plutonium or uranium-based device was used. But a well-contained test could yield no radioactive isotopes, experts say.

South Korean planes and ships were sent out immediately after the test to collect samples, as was a Japanese plane.

"Two days since the North's nuclear test, the commission has completed analyzing eight samples, but no radioactive isotopes have been discovered as of 15:00 Thursday," the commission said.

No changes had been recorded at 122 unmanned radiation monitoring systems across the country, it added.

North Korea said earlier this week that it had carried out an underground nuclear test, which it claimed involved a miniaturized but more powerful device.

Seismic activity triggered by the test at the Punggye-ri site in the north-east of the country was detected by several nations.