"It would pose a serious threat that would shake the international non-proliferation system from its foundation, and North Korea would be further isolated," Ban told journalists.

"It would bring about much more serious consequences than its missile test last month," he said.

The North, which says it has developed nuclear weapons, test-fired seven missiles on July 5, drawing the ire of the international community and criticism from the UN Security Council.

South Korea has stepped up monitoring of North Korea's nuclear activities amid news reports that the communist state may be preparing for an underground nuclear bomb test.

"At this moment, we cannot rule out the possibility (of North Korea preparing for a nuclear weapon testing)," Ban said in response to a question whether he had information confirming that preparations for such a test were under way in Punggye-ri in North Korea's northeastern county of Gilju.

"In close cooperation with other countries concerned, we have been watching the situation closely," he said.

The US television network ABC reported on Thursday that North Korea may be preparing to conduct an underground test of a nuclear bomb, attributing it to US officials.

Analysts here said the reclusive North might pretend to prepare a nuclear test to bluff the United States into looking afresh at stalled negotiations over North Korea's nuclear programme.

An imminent nuclear test was predicted in North Korea last year but no test occurred.

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