President Barack Obama vowed "firm action" alongside US allies against North Korea, as South Korea accelerated a planned upgrade to its offensive missiles, after Pyongyang ignited global fury with its third nuclear test.
China, North Korea's trade and financial lifeline, signed up to a UN Security Council statement accusing the communist state of standing in "grave violation" of UN resolutions amid global condemnation of Tuesday's blast.
The Council highlighted its threat made last month -- after North Korea breached UN resolutions by firing a long-range rocket -- to take "significant action" if Pyongyang went ahead with the nuclear test.
But North Korea, which analysts say has closed ranks under the young leadership of Kim Jong-Un, remained defiant after saying the underground test was forced upon it by US "hostility."
"The DPRK (North Korea) will never be bound to any resolutions," said Jon Yong-Ryong, first secretary of North Korea's mission in Geneva, berating the UN resolutions as "entirely unreasonable."
Any tougher UN action after years of sanctions against the recalcitrant North will depend on how far China is willing to push its ally. In his State of the Union address to Congress Tuesday, Obama stepped up the rhetoric.
"America will continue to lead the effort to prevent the spread of the world's most dangerous weapons," he said.
"The regime in North Korea must know that they will only achieve security and prosperity by meeting their international obligations."
North Korea alarmed friends and foes alike less than a day before Obama's speech when it carried out its latest nuclear test, which US and South Korean monitors said was much more powerful than the previous tests in 2006 and 2009.
Pyongyang boasted it had tested a "miniaturised" device, a claim that will fuel concerns it has moved closer to fitting a warhead on a ballistic missile.
Experts were assessing whether the test involved uranium, giving Pyongyang a new and easier-to-sustain method for nuclear fission alongside its depleted stocks of plutonium.
Obama said: "Provocations of the sort we saw last night will only isolate them further, as we stand by our allies, strengthen our own missile defence and lead the world in taking firm action in response to these threats."
Even China, which is keen to avoid the chaos that could ensue if the isolated North collapses, was stern in its condemnation of the test and summoned North Korea's ambassador in Beijing.
Chinese Foreign Minister Yang Jiechi discussed the test by phone with US Secretary of State John Kerry, the foreign ministry said, reiterating China's longtime stance of urging all sides to "avoid an escalation of the situation".
But Chinese social media users berated authorities for what they saw as the relatively mild response.
One commentator using the nickname Wuyuesanren slammed the idea that North Korea's nuclear program boosted China's security, likening Beijing's policy to "keeping a crazy dog to guard the house".
North Korea "simply doesn't trust China and is not willing to be inhibited by China", wrote Zhuanshengben in another post. "For China alone to emphasise China and North Korea's so-called friendship, this is the ultimate stupidity."
South Korea, which placed its US-backed military on alert after the test at North Korea's Punggye-ri site, said it would fast-track the development of longer-range ballistic missiles that could cover the whole of North Korea.
"We will speed up the development of ballistic missiles with a range of 800 kilometers (500 miles)," defence ministry spokesman Kim Min-Seok told reporters.
Last October South Korea reached a deal with the United States to almost triple the range of its missile systems -- with Seoul arguing it needed an upgrade to counter the North's missile and nuclear programmes.
Kim said the South would also speed up the deployment of a "kill chain" system capable of detecting, targeting and destroying North Korean missiles.
Following the test, the head of South Korea's intelligence agency warned that Pyongyang may well carry out a further test or a ballistic missile launch in the coming days or weeks.
The North itself insisted that any tightening of sanctions would trigger "even stronger second or third rounds of action."
All 15 Council members including China backed Tuesday's UN statement and said they would "begin work immediately on appropriate measures".
The test ignited yet another round of tension on the Korean Peninsula, where peace has never been formally declared since a 1950-53 war between the authoritarian north and pro-Western south.