"We must move from an unprincipled and unilateral policy of appeasement to a pro-active policy that results in enduring change," said Lee Myung-Bak of the main opposition Grand National Party (GNP).

The essential condition, he told a press conference with foreign correspondents, is "North Korea's complete nuclear dismantlement. A freeze is not enough."

Six-nation nuclear negotiations resume Thursday in Beijing amid reports the North will offer to freeze its Yongbyon reactor in exchange for oil supplies and other benefits.

Lee, 65, a former CEO of the Hyundai group and ex-mayor of Seoul, is the clear front-runner for the December election with an approval rating of over 40 percent.

The conservative GNP has traditionally been tougher on the North than the ruling left-leaning Uri party, especially since the North's nuclear test last October.

Pyongyang, in return, heaps abuse on the GNP. Official media described it Monday as a "group of traitors who inherited the 'traditions' of most despicable fascism and treachery."

Lee accused the government of a "business as usual" response to the North's July missile launches and October nuclear test. "This was a colossal failure of our national security policy and our citizens are under threat."

He described North Korean leader Kim Jong-Il as a "longstanding dictator" and said his continued posession of nuclear weapons is "not an option."

Lee urged Kim to liberalise his state-directed economy and scrap the nuclear programme, and said such a course would attract international aid that could push the North's per capita income to 3,000 dollars over the next decade.

Current per capita GDP is estimated at 1,700 dollars compared to almost 20,000 dollars in South Korea.

Lee said South Korea's relationship with the United States, which for the past 50 years has stationed tens of thousands of troops in the South, was "in crisis" because of the "deliberate policies and choices made by this government."

He called for a revitalised alliance, closer ties with Asian neighbours and a significant increase in Seoul's foreign aid.

Lee said former colonial power Japan was partly to blame for strains in the relationship with South Korea but added: "I am convinced the relationship between the two countries will improve."

He said he is also convinced that South Korea and China can be "very good neighbours" despite the latter's rapidly growing regional economic might.