It is the first time that Lee, who leads a conservative government, has publicly called for cuts in conventional arms, analysts said.
Lee reiterated that the South would help Pyongyang end its isolation and prosper if the communist state gave up its atomic weapons.
"If the North comes to such a decision, the [South Korean] government will push for a new programme for peace on the Korean peninsula," Lee said.
An international programme aimed at helping develop the North's devastated economy and improving the living standards of North Koreans would then be put into practice, he added.
Lee has previously offered huge long-term aid to the North in return for full nuclear disarmament, a linkage which Pyongyang angrily rejects.
The two Koreas are still technically at war since the Korean War of 1950-53 ended in an armistice, not a peace treaty.
More than 600,000 South Korean soldiers, backed by 28,500 US troops, are deployed on the Korean peninsula, confronting a potential threat from the North's 1.1 million-strong military.
Tension between the nations has risen since the conservative government in Seoul took office in February last year and pursued a firmer line with the North following a decade of engagement and two summits under his liberal predecessors.