The North failed to meet an end-of-2007 deadline in a six-country deal to release a complete accounting of its nuclear material and weaponry, as well as answer U.S. suspicions of having a secret programme to enrich uranium for weapons.
The deal is what the international community hopes will eventually lead to a complete nuclear disarming of the North.
"The North has shifted to blaming the South for what it has not been able to work out with the U.S.," said Choi Jin-wook, an expert on the North at the South's Korea Institute for National Unification.
Lee's government has said it would work closely with the United States and Japan, and its stance on North Korea puts South Korea closer to its traditional allies in trying to exert pressure on the North to force change.
The chief U.S. envoy to the North Korean nuclear talks is scheduled to arrive in Seoul later on Tuesday.
Lee has proposed an aid package for North Korea that would lift per capita income from a few hundred dollars a year to $3,000, provided it abides by the six-way nuclear deal.
The North called Lee's plan "piffle" and said it "will be able to live as well as it wishes without any help from the South as it did in the past".
Analysts said that China, the closest the North has to a major ally, would lean on the hermit state to prevent the situation on the Korean peninsula spinning out of control.
Beijing, already facing criticism for its handling of the crises in Tibet and Sudan, does not want North Korea to be another headache and spoil its hosting of the 2008 Summer Olympics, they said.
