North Korea says it has successfully conducted an atomic explosion test aimed at examining the power of its nuclear warheads.
State TV said the test elevated the country's nuclear arsenal and is part of its response to international sanctions following its earlier nuclear test and long-range rocket launch in January and February.
Pyongyang said it will continue to take efforts to strengthen the quantity and quality of its nuclear weapons.
South Korean president Park Geun-hye strongly condemned the test, saying in a statement that it showed the "fanatic recklessness of the Kim Jong Un government as it clings to a nuclear development".
Confirmation of a fifth nuclear test by Pyongyang came after international monitors detected unusual seismic activity near a north-eastern test site.
South Korea's Joint Chiefs of Staff said in a statement that "artificial seismic waves" from a quake measuring 5.0 were detected near the Punggye-ri test site.
A South Korean Defence Ministry official said Seoul detected an estimated explosive yield of 10 kilotons and assessed that it was from a nuclear test.
After the fourth test in January, South Korean policymaker Lee Cheol Woo said Seoul's National Intelligence Service told him an estimated explosive yield of six kilotons was detected.
The 5.0 magnitude earthquake is the largest of the quakes associated with North Korean nuclear tests, according to South Korea's weather agency. Artificial seismic waves measuring 3.9 were reported after the first nuclear test in 2006, and 4.8 was reported from its fourth test.
North Korea's nuclear tests are part of a push for a nuclear-armed missile that could one day reach the US mainland. A second nuclear test this year is a defiant response to Western pressure on Pyongyang to halt its nuclear ambitions. The country has previously conducted tests every three to four years.
North Korean leader Kim Jong Un (pictured below) has overseen a robust increase in the number and kinds of missiles tested this year. Not only has the range of weapons successfully tested jumped significantly, but the country is working to perfect new platforms for launching them - submarines and mobile launchers.
The longer ranges and mobile launchers give the North greater ability to threaten the tens of thousands of US troops stationed throughout Asia.
The seismic activity comes on the 68th anniversary of the founding of North Korea's government and days after world leaders gathered in China for the G20 economic summit.
The new test will lead to a strong push for tougher sanctions at the United Nations and further worsen already abysmal relations between Pyongyang and its neighbours. North Korea is already one of the most heavily sanctioned places on earth, and many question whether the penalties work.
North Korea is thought to have a handful of rudimentary nuclear bombs and has spent decades trying to perfect a multi-stage, long-range missile to eventually carry smaller versions of the bombs.
After several failures, it put its first satellite into space with a long-range rocket launched in December 2012, and has since had another successful launch.
Experts say ballistic missiles and rockets in satellite launches share similar bodies, engines and other technology. The UN calls the North's long-range rocket launches banned tests of ballistic missile technology.
It is unclear whether the North has achieved the technology needed to manufacture a miniaturised nuclear warhead that could fit on a long-range missile capable of hitting the US. Some analysts believe the country has the ability to arm shorter range missile with warheads.
On Tuesday, North Korea fired three medium-range Rodong missiles that travelled about 620 miles and landed near Japan, and last month, a missile from a North Korean submarine flew about 310 miles, the longest distance achieved by the North for such a weapon. This worried many South Koreans because submarine-based missiles are harder to detect before launch than land-based weapons.
Diplomacy has so far failed to stop North Korea's progress. Six-nation negotiations on dismantling Pyongyang's nuclear programme in exchange for aid were last held in late 2008 and fell apart in early 2009, when North Korea was led by Kim Jong Un's father, Kim Jong Il, who died in late 2011.
North Korea blames the US and South Korea for its nuclear programme, saying long-running "hostility" from Seoul and Washington to its government makes the development crucial for the small country's survival.