|North Korean leader Kim Jong Un inspects the completed Mirae Scientists Street, in this undated photo released by North Korea''s Korean Central News Agency (KCNA) in Pyongyang on October 21,.|
If confirmed, the test would be the fourth time Pyongyang has exploded a nuclear device.
Before Wednesday's announcement, which was made on state television, South Korea's meteorological agency said the North was likely to have conducted a nuclear test after a 5.1 magnitude earthquake was detected near a known test site in the secretive and isolated state.
South Korea's presidential office held an emergency meeting and later said the government would take all possible measures in response to the apparent test.
Our government strongly condemns North Korea ignoring repeated warnings from us and the international community and pushing ahead with the fourth nuclear test, which clearly violated the UN resolutions," Cho Tae-yong, a senior security official at the South Korean presidential office said.
The UN security council called an emergency meeting for Wednesday, diplomats said, as both neighbouring countries and world powers issued stern statements in response to the announcement.
China's foreign ministry said it had no advance knowledge of any test and added that it firmly opposed Pyongyang's action.
Ministry spokeswoman Hua Chunying said at a daily news briefing that China would work with the international community on the denuclearisation of the Korean peninsula.
The US government said it could not confirm a nuclear test but that it would respond appropriately to what it called North Korean "provocations" and that it would continue to protect its allies in the region.
"We have consistently made clear that we will not accept [North Korea] as a nuclear state," a State Department statement said.
Japanese Prime Minister Shinzo Abe called the announcement a threat to his nation's safety.
Abe told reporters: "We absolutely cannot allow this, and condemn it strongly."