Its most powerful explosion to date follows a test in January that prompted the UN Security Council to impose tightened sanctions on North Korea.
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.
US President Barack Obama, aboard Air Force One on his way home from Laos, said the test would be met with "serious consequences" and held talks with Park and with Japanese Prime Minister Shinzo Abe, the White House said, according to Reuters.
North Korea, which labels the South and the US 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.
North Korea said the test proved it is capable of mounting a nuclear warhead on a medium-range ballistic missile, which it last tested on Monday.