Their business-like summit largely steered clear of problems that clouded other meetings such as disputes stemming from Japan's 1910-1945 colonial rule over the Korean peninsula and a territorial row over desolate islets.
"It is unprecedented that leaders from Japan and South Korea see each other so often and agree to co-operate in broad areas like this," Aso said.
Free trade talks
Aso and Lee have met every month since October at diplomatic events.
The two countries are each other's third-largest trading partners after China and the United States. Japan-South Korea trade totalled $81.4bn in 2007, up 4.8 per cent from the previous year, according to Japan's foreign ministry.
Seoul's presidential office said the two countries would hold working-level talks on resuming free trade agreement negotiations that have been in limbo since 2004, primarily stalled by wrangling over tarriffs.
On Monday, the Japanese dailies Asahi and Yomiuri showed support for Aso at around 20 per cent, the lowest since he took office in September.
Lee's rating stands at around 23 per cent.