North Korea's reported second nuclear test on Monday was more powerful than its first in October 2006, Seoul's weather agency said.

It appeared to have taken place 10 to 15 kilometres northeast of the site of the first blast, the agency said.

"The magnitude of the quake, measured at 4.5 on the Richter Scale, was more powerful than the past one which was measured at 3.6," Yoo Yong-Gyu, a senior researcher at the Korea Meteorological Administration, told AFP.

"The seismological event took place at N 41.28 and E 129.13," some 10 to 15 kilometres northeast of the previous site, he said.

Pyongyang's state media said the country had "successfully" conducted an underground nuclear test but gave no location for the test.

It said Monday's second test was conducted on "a new higher level in terms of its explosive power and technology of its control".

The weather office reported its findings immediately to the presidential office.