North Korea warned last night that it has more in store after unleashing its third - and largest - nuclear test on Tuesday in a move swiftly condemned by China.
A statement from the Foreign Ministry in Pyongyang said the test was the "first response" to what it called US threats. "Second and third measures of greater intensity" would be taken if Washington continued its hostility.
China's Foreign Minister Yang Jiechi immediately summoned the North Korean ambassador Ji Jae-ryong for a formal protest warning that Beijing was "strongly dissatisfied and resolutely opposed" to the test.
Yang also told the envoy to "stop any rhetoric or acts that could worsen situations and return to the right course of dialogue and consultation as soon as possible", according to a Foreign Ministry statement.
United Nations Secretary General Ban Ki-moon and US President Barack Obama also condemned the test, which took place shortly before noon local time at the Punggye-ri test centre near North Korea's northeast border with China and Russia.
The test defied a UN move tightening sanctions against Kim Jong-un's regime three weeks ago. The UN Security Council, which met in New York on Tuesday, "strongly condemned" the nuclear test. The council statement made no direct reference to sanctions but demands for new measures were quickly made by the United States and its allies.
Pyongyang's KCNA news agency described the "perfect" test of a "smaller and lighter nuclear device". South Korean officials estimated the explosion at 6-7 kilotons - much larger than devices exploded in 2006 and 2009 but still relatively small. The atomic bomb dropped on Hiroshima at the end of the second world war was 10 kilotons.
International experts are trying to confirm that Pyongyang used highly-enriched uranium for the first time, instead of plutonium. Such a move would represent a second route to nuclear capability for North Korea - one that is easier to conceal and trade.
Kim Jong-un, who took power after the death of his father Kim Jong-il in December 2011, appears keen to exploit a moment of unusual political uncertainty.
New leaderships are preparing to take power in China and South Korea, while in Japan, the once-dominant Liberal Democratic Party is settling back into office. It represents a key early test, too, for new US Secretary of State John Kerry.
Chinese state media earlier warned that Pyongyang would pay a "heavy price" if it went ahead with the test.
"China's tolerance to North Korea is limited," said Peking University international relations professor Jia Qingguo , adding that Beijing may support new sanctions. "China will be unlikely to follow its tradition of rejecting the use of sanctions to solve problems. It is not China initiating the change in tactics, but North Korea pushing China to adjust its policies."
Additional reporting by Associated Press, Reuters