A special operation to begin “soon” would “reduce its target to ashes”, the military said in a statement.
On Friday North Korea also held a rally calling for the death of South Korea’s president, condemning his comments against the North.
Seoul said last week it had deployed more missiles against the North.
A defence ministry spokesman said the missiles had a range of 1,000km (625 miles) and could hit any of Pyongyang’s nuclear or missile sites.
The North’s statement directly criticised South Korean President Lee Myung-bak, the defence minister, analysts and “rat-like elements” including conservative broadcasters, who were accused of “destroying fair-minded public opinion.”
The statement, which was read out on state television, said that “the special actions of our revolutionary armed forces will start soon to meet the reckless challenge of the group of traitors”.
North Korean rhetoric has been getting sharper in recent days following its failed rocket on 13 April, reports the BBC’s Lucy Williamson.