"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 in a statement carried by the state-run Korean Central News Agency.
The North warned that Seoul's continued hard-line stance would only draw "a heavier blow and shameful destruction" on the South. In Seoul, Unification Ministry spokesman Kim Ho-nyeon said the government would issue a response later Friday.
The two Koreas technically remain at war because their brutal, three-year conflict ended in a truce, not a peace treaty, in 1953. The peninsula remains divided by a heavily fortified Demilitarized Zone, with thousands of troops stationed on both sides of the border.
Ties have warmed significantly over the past decade, with North Korean leader Kim Jong Il meeting with then-South Korean President Kim Dae-jung in a historic summit in 2000. The detente helped pave the way for first inter-Korean exchanges in 50 years.
But tensions have been high since Lee took office in Seoul nearly a year ago pledging to get tough with Pyongyang. He questioned the wisdom of his predecessors' "sunshine policy" of nurturing reconciliation by handing over aid to the nuclear-armed North unconditionally.
Pyongyang responded by cutting off all reconciliation talks with Seoul, suspending key joint projects and ratcheting up the rhetoric against a man they denounce as a "traitor" to Korean reunification.
"Never to be condoned are the crimes the Lee group has committed against the nation and reunification by bedeviling overnight the inter-Korean relations that had favorably developed amidst the support and encouragement of all the Koreans and ruthlessly scrapping the inter-Korean agreements," the North said Friday.
Earlier this month, the North's military accused the South of preparing to wage war and said it was prepared to respond to any southern aggression.
Seoul has denied plotting any attack on the North.
On Friday, the North declared all agreements on the Koreas' disputed western maritime border "nullified," raising the specter of a naval skirmish. Disputes over the border prompted two deadly clashes in 1999 and 2002.
The U.S.-led United Nations Command unilaterally drew the Yellow Sea border at the end of the war - but Pyongyang claims it should be redrawn farther south.
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Associated Press writer Jae-soon Chang contributed to this report.
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