* Test seen as North’s most powerful yet, possibly bigger than bomob dropped on Hiroshima
* Japan protests, sends jets to monitor for radiation
* China begins emergency radiation testing
* Test caused magnitude 5.3 earthquake
* Thought to have yield of 10 kilotons
The longer story
North Korea conducted its fifth nuclear test this morning. It set off a blast that was seemingly more powerful than the bomb dropped on Hiroshima, and said it could now put a warhead on a missile.
Leaders returning from the Asean summit in Laos were aghast. South Korean President Park Geun-hye 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.
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 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.
Its most powerful explosion to date follows a test in January that prompted the U.N. Security Council to impose tightened sanctions that increased North Korea’s isolation but failed to prevent it from accelerating weapons development.
North Korea tested medium-range missiles on Monday, which provoked an angry response from leaders gathered in China for the G20 summit. It's claims of being able to make a nuclear warhead small enough to fit that kind of missile have never been proved by outsiders.
Some of North Korea's neighbours were at a loss as to how to stop the isolated nation making more nuclear weapons.
"Sanctions have already been imposed on almost everything possible, so the policy is at an impasse," said Tadashi Kimiya, a University of Tokyo professor specialising in Korean issues.
"In reality, the means by which the United States, South Korea and Japan can put pressure on North Korea have reached their limits,” he said.
Japan’s Abe said such a nuclear test could not be tolerated. Japan’s foreign minister lodged a protest and Tokyo also sent two military jets to begin measuring for radiation.
China’s environment ministry began emergency radiation monitoring along its borders with North Korea in northeast China, state television reported.