North Korea has threatened South Korea with 'final destruction' during a debate on disarmament at the UN, as it said it could take "second and third steps" after a nuclear test last week.

"As the saying goes a new born puppy knows no fear of a tiger. South Korea's erratic behaviour would only herald its final destruction," North Korean diplomat Jon Yong Ryong told the meeting.

The threat came as Moscow opposed new economic sanctions on North Korea but said it would be ready to back measures to limit nuclear proliferation, a Russian official said on Tuesday.

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Pyongyang was widely condemned last week after its third nuclear test since 2006, defying United Nations resolutions and putting the country closer to a workable long-range nuclear missile

Without specifically referring to the nuclear test, Jon said North Korea had recently taken a "resolute step for self-defence", which he described as "strong counter-actions to a foreign aggressor".

"If the US takes a hostile approach toward the DPRK to the last, rendering the situation complicated, it (North Korea) will be left with no option but to take the second and third stronger steps in succession," he added, without elaborating.

His comments drew immediate criticism from other nations, including South Korea and Britain, whose ambassador Joanne Adamson said such language was "completely inappropriate" and the discussion with North Korea was heading in the wrong direction.

"It cannot be allowed that we have expressions which refer to the possible destruction of UN member states," she said.

US Ambassador Laura Kennedy said she found North Korea's statement profoundly disturbing.

"I also was particularly struck by the phrase 'heralding the destruction of the Republic of Korea' and find that language incredibly inconsistent with the goals and objectives that this body is intended to pursue," she said.