NORTH Korea tested a nuclear weapon yesterday, briefly shaking Asian markets and triggering an emergency UN Security Council meeting on the communist state&rsquo;s act of defiance.<br /><br />US President Barack Obama said Pyongyang&rsquo;s bid to develop nuclear weapons was a threat to international peace and security, and called on world leaders to condemn the test. China, traditionally seen as a friend of North Korean leader Kim Jong Il,&nbsp; said it was &ldquo;resolutely opposed to the test&rdquo;.<br /><br />Russia, which called the test a threat to regional security, said the massive blast was equal in power to the atom bomb that the US dropped on Nagasaki in World War Two, and about 20 times larger than North Korea&rsquo;s one kiloton test in 2006.<br /><br />And the Vienna-based Comprehensive Nuclear Test Ban Treaty Organisation said the magnitude of the blast caused an earthquake measuring 4.52 on the Richter scale.<br /><br />&ldquo;North Korea&rsquo;s nuclear and ballistic missile programs pose a grave threat to the peace and security of the world, and I strongly condemn their reckless action,&rdquo; Obama said at the White House. &ldquo;The United States and the international community must take action in response.&rdquo;<br /><br />The UN Security Council unanimously condemned the nuclear test, saying it was a &ldquo;clear violation&rdquo; of a previous resolution passed in 2006 after Pyongyang&rsquo;s first atomic test.<br /><br />Officials in Washington and Beijing said North Korea had given them around an hour&rsquo;s notice before detonating the device, but Japan &ndash; which sees itself as a prime target &ndash; said it had not received any warning.<br /><br />South Korea&rsquo;s benchmark Kopsi was rocked by the news, plunging as much as 6.3 per cent, although it later bounced back to trade around 1.3 per cent lower at 1,385.76.<br /><br />The nuclear test had little impact on Japanese markets, which have grown accustomed to the threat of North Korea.&nbsp; The benchmark Nikkei index added 1.3 per cent to close at 9,347.<br /><br />Martin Slaney, head of derivatives at GFT, said the test would &ldquo;lend an air of caution to trading&rdquo; on European markets today.