North Korea said on Friday that it "successfully" ttest fired a nuclear test, its fifth and the "most powerful" test so far.
Reports said that the country carried out a nuclear explosion test to check the capacity of a nuclear warhead.
"We will continue to strengthen our capacity to bolster our nuclear force," it said.
A South Korean Defence Ministry official was quoted by Yonhap news agency as saying that the military was examining details on what type of nuclear material was used.
An artificial quake of magnitude 5 was noticed around 9.30 a.m. at North Korea's main Punggye-ri nuclear test site.
The US Geological Survey said the tremor was a 5.3-magnitude earthquake.