South Korean nuclear experts, checking for contamination after North Korea's sixth and largest nuclear test, said on Friday they have found minute traces of radioactive xenon gas but that it was too early to specify its source.

The Nuclear Safety and Security Commission (NSSC) said it had been conducting tests on land, air and water samples since shortly after North Korea's nuclear test on Sunday.

The statement said the commission was analysing "how the xenon entered South Korean territory and will make a decision at a later time whether the material is linked to North Korea's nuclear test".

Xenon is a naturally occurring, colourless gas that is used in manufacturing of some sorts of lights. But the NSSC said it had detected xenon-133, a radioactive isotope that does not occur naturally and which has in the past been linked to North Korea's nuclear tests.

There was no chance the xenon "will have an impact on South Korea's territory or population", the statement said.