North Korea announced it had conducted its fifth nuclear test, hours after the US Geological Survey's seismic monitors detected a blast near the secretive country's nuclear test site.
North Korea's state TV said the North was now capable of mounting nuclear warheads on ballistic rockets.
South Korean President Park Geun-hye, in Laos after a summit of Asian leaders ended there on Thursday, said North Korean leader Kim Jong-un was showing "maniacal recklessness" in completely ignoring the world's call to abandon his pursuit of nuclear weapons.
'Slightly smaller than Hiroshima'
South Korea has used previous seismic readings to estimate the power of the nukes being tested, but many variables, explosion-depth for example, make the estimates unreliable. The size of this quake suggests that this is the largest of the five nuclear detonations the nation has conducted since 2006.
The Director General for Earthquake and Volcano at South Korea's Meteorological administration, Kim Nam-ouk, said, "The energy of the magnitude 5.04 is double that of the fourth nuclear test (by North Korea)."
"We analysed that the seismic activity blast was slightly smaller than Hiroshima atomic bomb," Kim told reporters.
US President Barack Obama, aboard Air Force One on his way home from Laos after meeting with Park and with Japanese Prime Minister Shinzo Abe, said the test would be met with "serious consequences."
China, North Korea's only major diplomatic ally, said it was resolutely opposed to the test and urged Pyongyang to stop taking any actions that would worsen the situation.
North Korea, which labels the South and the United States as its main enemies, said its "scientists and technicians carried out a nuclear explosion test for the judgment of the power of a nuclear warhead," according to a report from its official KCNA news agency.
In January, a 5.1 magnitude earthquake in the area had been caused by what North Korean President Kim Jong-un called “the H-bomb of justice.”
This comes on the heels of three ballistic missile tests conducted by North Korea last week, as world powers gathered for a G20 meeting in China.