North Korea conducted an underground nuclear test on Monday, stoking tension across the regional economic powerhouse of East Asia and prompting U.N. Security Council members to call an emergency meeting.
British Prime Minister Gordon Brown called North Korea a danger to the world, and Russia said the hermit state's defiant act was about equal in power to the U.S. atom bomb dropped on the Japanese city of Nagasaki in 1945 at the end of World War Two.
The latest test will further confound the international community, which has for years tried a mixture of huge aid pledges and tough economic sanctions to persuade the impoverished North to give up efforts to build a nuclear arsenal.
It is already so isolated there is little left with which to punish an autocratic government that has long been willing to take dealings with the outside world to the brink.
At home, its leaders repeatedly stress the threat from a hostile United States to justify heavy spending on the military that keeps them in power but which has meant deepening poverty, at times famine, for most of the rest of its 23 million people.
(North Korea) successfully conducted one more underground nuclear test on May 25 as part of the measures to bolster up its nuclear deterrent for self-defense in every way, the official KCNA news agency said.
The country's first test, in October 2006, was considered to have been relatively weak, about 1 kiloton, suggesting design problems. Russia's military said the latest test had a force of up to 20 kilotons.
Analysts said North Korea will want the test to raise its leverage in any negotiations with the United States.
The test also comes as speculation has mounted that leader Kim Jong-il, his health uncertain after reports of a stroke last year, wants to strengthen an already iron grip on power so he can better secure the succession for one of his three sons.
News of the test rattled South Korea's financial markets, sending the main stock market index down more than six percent at one stage on worries by some that investors would flee.
Ratcheting up tensions further, North Korea test-fired three short-range missiles just hours later, the Yonhap news agency reported.
The nuclear test was another blow to South Korean markets, already unsettled by fears of domestic unrest after former President Roh Moo-hyun, who had been questioned over his links to a corruption scandal, jumped to his death at the weekend.
But the decline was short-lived and analysts said investors were used to the North's repeated saber-rattling, even as it became more aggressive, and would likely panic only if there was military conflict on a peninsula where 2 million troops face each other across one of the world's most heavily armed borders.
U.N. TO MEET
The U.N. Security Council will hold an emergency meeting later on Monday in the wake of the nuclear test, Russia's ambassador to the United Nations said.
Itar-Tass news agency quoted Russia's defense ministry as saying the blast was up to 20 kilotons, about the same size as the U.S. atomic bomb dropped on Nagasaki in 1945.
Russia, the United States, Japan and South Korea all expressed alarm at the test.
U.S. President Barack Obama said it was a matter of grave concern to all nations and warranted action by the international community.
European Union foreign policy chief Javier Solana called the test irresponsible but, like Obama, did not specify what actions the international community might take.
North Korea's actions are more for international audience, especially the United States, Koh Yu-hwan, Dongguk University professor of North Korea studies, said.
Hawkish military elements of the North Korean leadership want a stronger nuclear deterrence and status as a nuclear power before going to the negotiating table with the U.S.
North Korea had for weeks threatened to conduct the test in response to tighter international sanctions following its April launch of a rocket, widely seen as a disguised long-range missile that violated U.N. resolutions.
Following the tightened sanctions, Pyongyang also said it would no longer be a party to six-country talks on giving up its nuclear weapons program.
North Korea's strategic objective hasn't changed. That objective is to win the attention of the Obama administration, to push the North Korea issue up the agenda, said Xu Guangyu, a researcher at the China Arms Control and Disarmament Association.
Xu said China, the nearest Pyongyang has to a powerful ally, might support a U.N. resolution censuring North Korea but would not back much harsher sanctions.
China's goal is to ensure that the six-party talks process does not fall apart. Stricter sanctions are not going to achieve that objective, Xu said.
Some analysts have said the test may also be aimed at boosting the position at home of leader Kim Jong-il.
Several say Kim, who succeeded his father to create the world's first communist dynasty, may be trying to secure the succession for one of his three sons and that a nuclear test in defiance of world opinion could help him win support from his hardline military to do so.
North Korea can only be hawkish this time, because time's running out for Kim Jong-il. With the dire economy, North Korean public sentiment can go against succession, said Jang Cheol-hyeon, a researcher at the Institute for National Security Strategy and a former North Korean official.
(Reporting by Kim Junghyun, Rhee So-eui, Park Jongyoun, Marie-France Han, Jon Herskovitz, Miyoung Kim, Angela Moon, Kim Yeon-hee, Yoo Choonsik and Jack Kim in SEOUL, Chris Buckley in BEIJING, Linda Sieg in TOKYO, Connor Humphries in MOSCOW, Editing by John Chalmers)