It warned that the North did not engage in "empty talk"
"The Lee Myung-bak group should bear in mind that our guns and bayonets ... are aimed at their throats"
North Korean statement
An earlier statement released on Saturday labelled Lee a "traitor" and said his government had "opted for confrontation, denying national reconciliation and cooperation".
As a result, the North said, "our revolutionary armed forces are compelled to take an all-out confrontational posture to shatter them."
That threat was considered particularly pointed and serious because it was delivered by a uniformed military officer - flanked by military unit flags - instead of the usual television newsreader.
Both statements follow months of verbal attacks on the Lee's conservative government which has vowed to get tough on the North and end years of unconditional aid.
Late last year, with relations rapidly deteriorating, Pyongyang announced it was closing most border traffic between the two states and suspending several cooperation projects.
|South Korea says its armed forces have been put on heightened alert [EPA]
Despite the raised military alert, Won Tae-jae, a spokesman for the South Korean defence ministry, told a news conference that "much of the comments are the usual rhetoric and arguments that have been issued previously".
South Korea has denied it has any plans to invade.
North and South Korea remain officially at wear never having signed a formal ceasefire ending the 1950-53 Korean War.
Both sides have deployed thousands of soldiers along the so-called "demilitarised zone" that divides the two countries, with almost 30,000 US militray personnel based in South Korea as well.
The latest statements from the North came after a US expert returning from talks in Pyongyang said the reclusive state had "weaponised" enough plutonium for four to five nuclear weapons.
Selig Harrison told Reuters news agency that senior North Korean officials had told him that 30.8kg of plutonium it had listed as part of a preliminary disarmament deal had been incorporated into warheads or other weapons.
Harrison said the comments could not be immediately verified, but they underscored a hardening of the state's position, and could pose as an even greater obstacle to international demands that the North dismantle its nuclear programme.