Obama vows to get tough with North Korea in speech

WASHINGTON (AFP) - President Barack Obama vowed on 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 "miniaturised" device, a claim that will fuel concerns it has moved closer to fitting a warhead on a ballistic missile.