A government-affiliated agency says that based on seismic wave analysis, the tremor recorded Monday morning in the North Korean county of Hwadae in North Hamgyeong Province was clearly manmade and caused by a large explosion.

The Korea Institute of Geoscience and Mineral Resources says, however, that it's still trying to confirm whether the quake was indeed a nuclear test as Pyongyang claimed.

It said the explosion apparently occurred several hundred meters below ground with an effect similar to detonating four to eight hundred tons of TNT.

Meanwhile, the Korea Institute of Nuclear Safety says it's working to confirm any radioactive emissions from the testing.