Seoul: North Korea on Friday said it "successfully" conducted a nuclear test, its fifth and the "most powerful" test so far, Yonhap news agency reported.
The North Korean Central TV Broadcasting Station said 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 analysing details on what type of nuclear material was used.
An artificial quake of magnitude 5 was detected 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.
The South Korean Defence Ministry official said the five-magnitude tremor felt after the test put the yield of this test at about 10 kilotons, which is believed to be Pyongyang's most powerful nuclear detonation to date.
The estimated yield of the fourth nuclear test was six kilotons.
A five-magnitude artificial tremor was also recorded after Pyongyang's fourth nuclear test on January 6 at Punggye-ri where all of the four North Korea underground nuclear tests had been carried out since 2006.
According to Yonhap, North Korea conducted the nuclear test to mark the country's 68th National Day on September 9.
It marked the fifth nuclear detonation by North Korea following the first in October 2006, the second in May 2009, the third in February 2013 and the fourth in January this year.
South Korean Prime Minister Hwang Kyo-ahn called an emergency meeting of the National Security Council (NSC) following the test, the presidential office Cheong Wa Dae said.
President Park Geun-hye, who is on an overseas trip to Laos, condemned the test, saying that its provocation would only invite stronger international sanctions, deeper isolation and hasten its self-destruction.
According to Yonhap, Park also held a telephone conversation with US President Barack Obama over the nuclear test.
Meanwhile, Japanese Prime Minister Shinzo Abe said North Korea's nuclear test was "unacceptable" and would be protested.
"It is simply unacceptable. We must strongly protest against it," Abe said, adding that Japan would liaise closely on the matter with the US and South Korea.
Pyongyang has conducted a series of ballistic missile launches since top North Korean leader Kim Jong-un gave an order on March 15 to test a nuclear warhead and ballistic rockets capable of carrying the warhead "in a short time".