President Barack Obama vowed Tuesday to take "firm action" with US allies against North Korea and South Korea accelerated a planned upgrade to its offensive missiles after Pyongyang's third nuclear test.

China, North Korea's trade and financial lifeline, signed up to a UN Security Council declaration 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, the 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, Obama stepped up the rhetoric.

"America will continue to lead the effort to prevent the spread of the world's most dangerous weapons," the US president 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 "miniaturized" device, a claim that will fuel concerns it has moved closer to fitting a warhead on a ballistic missile.

Experts are assessing whether the test involved uranium, giving Pyongyang a new, 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 defense and lead the world in taking firm action in response to these threats."

South Korea, which placed its US-backed military on alert after the test at North Korea's Punggye-ri nuclear test, 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)," Defense Ministry spokesman Kim Min-Seok told reporters.

In October last year, 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 programs.

Kim said the South would also speed up the deployment of a "kill chain" system capable of detecting, targeting and destroying North Korean missiles.

"The military is closely watching the North in case of further provocative acts," he said.

Following North Korea's nuclear 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.

All 15 Council members including China backed Tuesday's UN statement and said they would "begin work immediately on appropriate measures."

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, summoning North Korea's ambassador in Beijing to protest the test.

The test ignited yet another round of tension on the Korean Peninsula, where peace has never been formally declared since a war split it between the authoritarian north and pro-Western south in the 1950s.

The North also appeared keen to broaden the conflict, insisting that any tightening of sanctions would trigger "even stronger second or third rounds of action."

The totalitarian country mobilized public opinion on its side, with state television showing footage of cheering, clapping citizens watching the test being announced on giant TV screens in the capital Pyongyang.

burs-jit/dan