North Korea conducted its fifth and biggest nuclear test Friday, a move that drew a harsh and immediate response from its southern neighbors.
South Korean President Park Geun-hye, in Laos after a summit of Asian leaders, said Kim was showing "maniacal recklessness" in completely ignoring the world's call to abandon his pursuit of nuclear weapons.
The blast, on the 68th anniversary of North Korea's founding, was more powerful than the bomb dropped on Hiroshima, according to some estimates, and was received widespread condemnation, including from China, Pyongyang's main ally.
North Korea said it had also mastered the ability to mount a warhead on a ballistic missile.
Under 32-year-old Kim Jong Un, North Korea has accelerated the development of its nuclear and missile programs despite U.N. sanctions that were tightened in March.
U.S. 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 said it was resolutely opposed to the test and urged Pyongyang to stop taking any actions that would worsen the situation. It said it would lodge a protest with the North Korean embassy in Beijing.
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 its official KCNA news agency.
It said the test proved North Korea was capable of mounting a nuclear warhead on a medium-range ballistic missile, which it last tested on Monday when Obama and other world leaders were gathered in China for a G20 summit.
Pyongyang's claims of being able to miniaturize a nuclear warhead have never been independently verified.
It was not clear whether Pyongyang had notified Beijing or Moscow of its planned nuclear test. Senior officials from Pyongyang were in both capitals this week.
China's official Xinhua news agency, in a commentary after North Korea confirmed the test, said it was shocking and unwise and would only "add oil to the flames."
However, the statement also criticized South Korea's recent provocative actions, specifically the deployment of Terminal High Altitude Area Defence (THAAD) anti-missile system in the South.
"Not long along, South Korea ignored the strong opposition of neighboring countries and decided to deploy the THAAD system, which is diametrically opposed to efforts to maintain peace and stability on the peninsula, has seriously damaged regional strategic balance and caused a rise in tensions on the peninsula," read the statement.
Meanwhile, Russia strongly condemned North Korea's latest nuclear test on Friday, saying it threatens peace and security on the Korean peninsula and in the Pacific region.
"We insist that the North Korean side stop its dangerous escapades and unconditionally implement all resolutions of the United Nations Security Council," Russia's Foreign Ministry said in a statement.
NATO Secretary-General Jens Stoltenberg echoed those comments, telling Pyongyang it must drop all nuclear and ballistic missile activities.
The U.N. nuclear watchdog and the European Union's foreign policy chief both said the latest nuclear test was a violation of United Nations resolutions.