[The North's Committee for the Peaceful Reunification of Korea] warned that Lee's stance would only draw "a heavier blow and shameful destruction."
"The group of traitors has already reduced all the agreements reached between the north and the south in the past to dead documents," the committee in charge of inter-Korean affairs said early Friday in a statement carried by the state-run Korean Central News Agency.
Tensions between Seoul and Pyongyang have increased since Mr. Lee ended the previous administration's "sunshine policy" of unconditional aid to North Korea. Lee's policy is meant to get concessions from Pyongyang, particularly in regards to North Korea's nuclear weapons program.
The BBC adds that North Korea also criticized the South earlier this week over Seoul's appointment of Hyun In Taek as the new South Korean unification minister. Pyongyang said the appointment of Mr. Hyun was meant to increase hostilities between the Koreas. But despite the latest threats from the North, Seoul has responded calmly.
"Our government expresses deep regret," said Kim Ho-Nyoun, spokesman for South Korea's unification ministry, which handles cross-border affairs.
"We urge North Korea to accept our call for dialogue as soon as possible," he said.
The statement itself does not carry much weight because North Korea has cut off almost all contacts with the South over the past year in anger at Lee's tough stance. The North, which doesn't like to be ignored, has been lashing out at Lee for months, only to find its vitriolic barbs mostly ignored by Seoul. The latest statement may reflect its frustration. An armistice that marked the end of hostilities of the 1950-53 Korean War is not affected because South Korea was not a signatory.
Reuters notes that in order to further raise tensions, North Korea could opt for military action along the two countries' disputed Yellow Sea border. But Reuters adds that last time shots were exchanged between their two navies in 2002, North Korea was "badly outgunned" by South Korea's superior navy, and the technological divide between the two has only increased since then.
"North Korea seems to be throwing a tantrum to seek attention from an apathetic South Korea," Ryoo Kihl Jae, a professor at Seoul-based University of North Korean Studies, said by telephone. South Korea should discern North Korea's "real intentions."
The government in Pyongyang may be trying to get the attention of President Barack Obama's new administration in Washington, which hasn't shown signs of placing North Korea's nuclear weapons challenge at the top of its must-do list in foreign affairs.
"I think this has a lot to do with Barack Obama and not much to do with South Korea," Lance Gatling, a Tokyo-based consultant, who is an expert on North Korea's weapons of mass destruction, said in an e-mailed exchange.
The North's latest round of saber-rattling comes amid reports that its leader, Kim Jong Il, appears to have recovered from what the US government believes to have been a stroke. Reuters writes that US officials now believe Mr. Kim has returned to active leadership of North Korea.
"Kim appears to be in the saddle right now, making key decisions for his government," the official said, speaking on condition of anonymity.
"He suffered some health setbacks several months ago, but, at least for now, those problems don't seem to be affecting his political position in North Korea."
The reemergence of Kim, who has not announced a successor, may mitigate concerns about the consequences of a power vacuum in the North. But Reuters reports that The Council on Foreign Relations warned earlier this week that the US must prepare for "sudden, destabilizing changes" in North Korea.
The Council on Foreign Relations said that although North Korea defied predictions in the 1990s that it would collapse after the death of its founder, economic meltdown and a deadly famine, the state remains weak and vulnerable.
Change scenarios ranged from an orderly transfer of power from leader Kim Jong-il to a successor to a possibly violent struggle for power between military factions to a breakdown in political authority that would sow chaos in a country believed to have nuclear, chemical and biological weapons and with millions of armed troops, it said.
"The stakes are simply too high and the risks too great for U.S. policymakers to assume that this will not happen any time soon or that very little can usefully be done in advance," said the report.
The report warned that "Kim Jong Il's condition may actually be much worse than press reports suggest and that his capacity to govern – if it hasn't already been seriously compromised – may be short lived."