North Korea warned that there would be no change in inter-Korean relations as long as Lee Myung Bak is South Korea's president, accusing him of "unethical rowdyism" for his policies after the death of Kim Jong Il.
Mr Lee's "madcap sabre-rattling" along the demilitarised zone between the two countries and his decision to limit South Korean visits to Kim's funeral were provocations "baffling human imagination," the Committee for the Peaceful Reunification of Korea said in a statement yesterday.
"This glaringly shows what rude political gangsters the group is and what a despicable immoral and depraved horde it is," the statement said of Mr Lee's government.
"It is clear that there is nothing to expect from the inter-Korean relations and the nation will suffer only calamities and misfortune as long as this guy stays in office."
The statement was the second in two days that said the North would have no dealings with Mr Lee's government, and came as the Korean Central News Agency (KCNA) reported that Kim Jong Un was appointed supreme commander of the Korean People's Army.
KCNA cited a decision taken on Friday at a meeting of the Politburo of the Central Committee of the Workers' Party of Korea.
Both Kim Jong Un and Mr Lee face leadership tests that could shape their attitude toward engagement. Kim Jong Un needs to cement his grip on power in a country where the UN says one-third of the children are physically stunted from a lack of nutrition.
Mr Lee and his ruling party, which rolled back the "Sunshine Policy" of engagement with the nuclear-armed North, have dropped in opinion polls ahead of elections later this year.
"Kim Jong Un needs to prove himself in launching his new regime -- and an economic measure would be the most efficient way of doing that," said Yang Moo Jin, a professor of North Korean politics at the University of North Korean Studies in Seoul. "What's more hard-hitting for North Koreans than policies that affect how they'll be able to put food on the table?"
Mr Kim's appointment as leader of the army signals North Korea's intent to carry on its military-first, or "Songun", policy, which gives priority in allocating resources and in state affairs to the army, Mr Yang said.
In his annual address today, Mr Lee will take advantage of the transition in the North and announce a more conciliatory stance, said Kim Young Yoon of the Seoul-based Korea Institute for National Reunification.
The opposition has blamed Mr Lee for escalating tensions, saying his tough stance provoked hostilities that killed 50 South Koreans in 2010.
"The current mood in South Korea is to take advantage of the North's regime change and improve inter-Korean relations," Mr Kim said.
"The easiest way to do that would be to call for high-level meetings to make way for resumed six-party talks," he added.
South Korea ordered a "low-level" alert after Kim Jong Il's death and expressed "sympathy" with the North Korean people, while limiting the number of its citizens who could travel to Pyongyang on condolence visits.
Mr Lee said the measures were meant to signal that his country wasn't hostile toward the North, while Pyongyang issued threats of "unpredictable catastrophic consequences" over the South's restrictions on visits.