North Korea defied international warnings Tuesday and carried out its third nuclear test, drawing immediate condemnation from leaders around the globe.
Pyongyang said the "successful" test was in response to what it called the "reckless hostility" of the United States, which has led the global charge toward expanding sanctions against the communist state.
State media said the underground test used a lighter, smaller nuclear bomb with greater explosive force than previous tests - raising fears Pyongyang has achieved a breakthrough in miniaturizing the technology.
Following the test, the North's Foreign Ministry defiantly warned of unspecified additional measures. South Korea has already placed its military on alert and hinted at the possibility of additional North Korean nuclear tests or missile launches on Tuesday.
The U.N. Security Council is holding an emergency meeting later Tuesday in New York to discuss the test, which prompted an outpouring of global criticism.
U.N. Secretary General Ban Ki-moon immediately condemned the "deeply destabilizing" test, calling it a "clear and grave violation" of international sanctions.
U.S. President Barack Obama called the test "highly provocative," saying it threatens U.S. security and international peace. He called for "swift and credible" action by the international community.
China, North Korea's main ally, expressed what it called "firm opposition" to the test. It also took the rare step of summoning the North Korean ambassador to let Pyongyang know it was "strongly dissatisfied and resolutely opposed" to the test.
North Korea for weeks had threatened to carry out its third nuclear test in retaliation for United Nations sanctions that were expanded in response to a December long-range rocket launch.
Initial reports suggest North Korea's latest nuclear test was stronger than its previous tests in 2006 and 2009. A South Korean Defense Ministry spokesperson said the explosion generated 6-7 kilotons.
Security analyst Michael McKinley with the Australian National University says it is important to determine in the coming days whether North Korea used plutonium or highly-enriched uranium to conduct the test.
"The crucial evidence here will be whether they used plutonium as they did in the other two [nuclear tests], or whether they've used uranium in this one. That would represent of course that they've got a secret uranium enrichment program," he said.
In 2009, Pyongyang announced it would begin enriching uranium, possibly giving it a second way to make fuel for nuclear weapons. Experts say uranium-enrichment programs are more sustainable and easier to hide in secret facilities.