North Korea has conducted its third nuclear test, defying UN warnings and taking a crucial step towards a weapon capable of striking the United States.
It claimed the atomic test was merely its "first response" to what it called US threats, and said it will continue with unspecified "second and third measures of greater intensity" if Washington maintains its hostility.
The underground test, which set off powerful seismic waves, drew immediate condemnation from Britain, Washington, the UN and others. Even its only major ally, China, summoned the North's ambassador for a dressing-down.
Monitoring stations in South Korea detected an earthquake in the North with a magnitude of 4.9 which corresponds to an estimated explosive yield of 6-7 kilotons. The yields of the North's 2006 and 2009 tests were estimated at 1 kiloton and 2 to 6 kilotons. By comparison, US nuclear bombs that flattened Nagasaki and Hiroshima during the Second World War were estimated at 13 kilotons and 22 kilotons.
President Barack Obama said nuclear tests "do not make North Korea more secure." Instead, North Korea has "increasingly isolated and impoverished its people through its ill-advised pursuit of weapons of mass destruction," he said.
In an emergency session the UN Security Council unanimously said the test poses "a clear threat to international peace and security" and pledged further action. The test was a defiant North Korean response to UN orders that it shut down its atomic activity or face more sanctions and international isolation. It will probably draw more sanctions from the United States and other countries at a time when North Korea is trying to rebuild its moribund economy and expand its engagement with the outside world.
Several UN resolutions bar North Korea from conducting nuclear or missile tests because the Security Council considers Pyongyang a would-be proliferator of weapons of mass destruction and its nuclear testing a threat to international peace and stability. North Korea is estimated to have enough weaponised plutonium for four to eight bombs.
North Korea dismisses that as a double standard, and claims the right to build nuclear weapons as a defence against the United States, which has been seen as enemy No. 1 since the 1950-53 Korean War. The US stations more than 28,000 troops in South Korea to protect its ally.
The test is the first since young leader Kim Jong Un took power of a country long estranged from the West. It will be portrayed in North Korea as a strong move to defend the nation against foreign aggression, particularly from the US
The UN Security Council recently punished North Korea for a rocket launch in December that the UN and Washington called a cover for a banned long-range missile test.