Speaking at the UN Conference on Disarmament in Geneva, North Korean diplomat Jon Yong Ryong said: "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."
Without specifically referring to the nuclear test, Mr 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 towards North Korea, rendering the situation complicated, we will be left with no option but to take the second and third stronger steps," 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". She added: "It cannot be allowed that we have expressions which refer to the possible destruction of UN member states."
US Ambassador Laura Kennedy said she found North Korea's statement profoundly disturbing. She added: "The language is incredibly inconsistent with the objectives this body is intended to pursue."