The isolated state on Friday set off its most powerful nuclear explosion to date, saying it had mastered the ability to mount a warhead on a ballistic missile, ratcheting up a threat that its rivals and the United Nations have been powerless to contain.

A US special envoy met with Japanese officials on Sunday and said later the United States may launch unilateral sanctions against North Korea, echoing comments by US President Barack Obama on Friday in the wake of the test.

"The group of Obama's running around and talking about meaningless sanctions until today is highly laughable, when their 'strategic patience' policy is completely worn out and they are close to packing up to move out," state-run news agency cited a North Korean foreign ministry spokesman as saying in a statement later on Sunday.

"As we've made clear, measures to strengthen the national nuclear power in quality and quantity will continue to protect our dignity and right to live from augmented threats of nuclear war from the United States," KCNA added.

The UN Security Council denounced North Korea's decision to carry out the test and said it would begin work immediately on a resolution.

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