North Korea could be preparing for a new type of nuclear test, a South Korean defence official said on Thursday.

The South Korean Army said they were closely monitoring the North's moves and considering all possibilities amid the possibility of a new test that could differ from the five former ones that have been conducted.

But an official did not specify if the difference lies in the scale of the detonation, media reported.

The confirmation comes after 38 North, a US-based think tank, reported an increase in activities at the North Korean nuclear test centre of Punggye-ri, which points to intense preparations for a new test.  

Earlier, analysts from the think tank had said that the Punggye-ri test site seemed to have been prepared to support an underground nuclear test of an explosive yield of up to 282 kilotonnes, over 10 times the power estimated for the last nuclear test carried out in September.

This has led some experts to surmise that North Korea could detonate multiple devices simultaneously in order to collect more data that could help speed up its nuclear programme.