The official Korean Central News Agency (KCNA) carried the remarks by the North's Joint Chief, General Ri Yong-Ho, during a national meeting on the eve of the birthday of the communist country's armed forces.
"The revolutionary armed forces of the DPRK (North Korea) are fully ready to frustrate any provocation of the aggressors at a single blow," Ri said.
"They will mobilise all means including the nuclear deterrent... should the US imperialists and the South Korean puppet warmongers dare intrude into the inviolable sky, land and seas of the DPRK even 0.001mm," he said.
He added that "a grave situation is now prevailing on the Korean Peninsula" because of the neighbouring South and its ally the US, who he said wanted to start a war.
The harsh rhetoric comes as tensions between the two Koreas are high over the sinking of a South Korean warship in March near the disputed sea border in the Yellow Sea with a loss of 46 lives.
Seoul has been careful not to point the finger directly at the North over the incident in the Yellow Sea, which has stoked already tense ties, and Pyongyang has denied it was to blame.
The tensions prompted US Secretary of State Hillary Clinton to say she hoped there would be "no miscalculation" that could spark a new war between the Koreas.