A defiant North Korea on Tuesday staged its most powerful nuclear test yet and warned of "stronger" action to follow if the ensuing wave of global condemnation translated into tougher sanctions.

Global powers, including Pyongyang's sole major ally China, denounced the test which the North said was of a "miniaturised" device -- a claim that will fuel concerns it has moved closer to fitting a warhead on a ballistic missile.

The isolated state said its third test, after previous detonations in 2006 and 2009 that triggered a raft of UN sanctions, was a direct riposte to US "hostility".

The UN Security Council was expected to debate new measures when it meets on Tuesday morning in New York, with the United States and its allies likely to push hard for China to get tough with its erratic ally.

In what amounted to a pre-emptive warning, North Korea's foreign ministry said Tuesday's test was only a "first" step and that any tightening of sanctions would trigger "even stronger second or third rounds of action".

South Korea's spy agency predicted the North might carry out another nuclear test or ballistic missile launch in coming days or weeks.

Confirmation of the test from the North's state media came nearly three hours after seismic monitors detected an unusual tremor at 0257 GMT in the area of the Punggye-ri nuclear test site in the northeast.

Analysts said the timing appeared to be an attention-grabbing calculation from a state well versed in provocative acts, coming just ahead of US President Barack Obama's State of the Union address at the start of his second term.

Obama denounced the test and called for a "swift" and "credible" international response as the Security Council readied to meet in emergency session.

China, whose trade and aid are a life-support to impoverished North Korea, expressed "firm opposition" to the nuclear test but stopped short of threatening any punitive action.

The foreign ministry in Beijing appealed for calm from "all parties" and said the situation should be resolved through dialogue.

China's leverage over Pyongyang is limited, observers say, by its fear of a North Korean collapse and the prospect of a reunified, US-allied Korea directly on its border.

Neighbours Japan and South Korea stressed the threat to their own national security, while Russia, NATO and the European Union all condemned the test as illegal and a flagrant violation of UN resolutions.

It was the North's first nuclear test since its new, youthful leader Kim Jong-Un took over from his late father Kim Jong-Il.

Security analysts said it sent an unequivocal message of intent on the back of a successful long-range rocket launch in December.

"The launch and the test are empirical evidence that Kim and his regime have no intention of negotiating away the North's nuclear programme any time soon," said Paul Carroll, programme director at the US-based Ploughshares Fund.

On a technical level, experts will be hungry to know if North Korea has switched from plutonium to a new and self-sustaining nuclear weaponisation programme using uranium.

The KCNA statement did not specify what fissile material was used, but noted that the test's success had provided the North with a "diversified" nuclear deterrent.

The North has substantial deposits of uranium ore and it is much easier to secretly enrich uranium, which can be done with centrifuges rather than the nuclear reactor required for plutonium enrichment.

Tuesday's explosion had a yield of six to seven kilotons, said South Korean defence ministry spokesman Kim Min-Seok, significantly more than the 2006 and 2009 tests which both used plutonium.

The explosive yield compared with 15 kilotons in the world's first atomic bomb dropped by the United States on Hiroshima in 1945.

The latest test throws down a stark security and diplomatic challenge to Obama as well as to new Chinese leader Xi Jinping.

Paik Hak-Soon, a North Korea expert at the Sejong Institute in Seoul, said Kim Jong-Un was intent on triggering a crisis that would force the international community to negotiate on his terms.

"The UN is running out of options and probably knows new sanctions would only have a limited impact," Paik said.

"The only real option for curbing further provocation is starting a dialogue with the North, but that will be very difficult given the domestic political pressure on leaders in the US, South Korea and Japan."

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