South Korean President Lee Myung-bak spent the day in Tokyo for meetings with Japanese Prime Minister Taro Aso — a month after Pyongyang's underground nuclear test ratcheted up tensions in the region.

The leaders, who spoke to reporters at Aso's residence, said they also discussed how to foster stronger economic cooperation. Aso said initial negotiations aimed at eventually forming a free trade agreement would be held on July 1.

"We need to demonstrate that North Korea has nothing to gain by conducting nuclear tests and missile launches," said Lee.

The North has intensified its rhetoric against the two countries and the U.S. in recent weeks, condemning the sanctions passed to punish Pyongyang for the nuclear test and raising concerns it will soon test-fire short- or mid-range missiles.

The visit is part of efforts by the Asian neighbors to improve their relationship, which has been troubled since Japan's 1910-1945 colonial rule of Korea. A set of tiny islands in the Sea of Japan that both claim has long been a source of friction, and South Korea has protested when Tokyo approves textbooks Seoul feels justify wartime transgressions.

But they have strong economic ties and share close alliances with the U.S., which maintains a large military presence in both countries.

Previous Japanese Prime Minister Yasuo Fukuda attended Lee's inauguration last year, after the South Korean made campaign promises to improve ties with Japan.

This is Lee's fourth visit to Japan as president. Aso, who visited Seoul in January, is expected to go again this year.

Aso and Lee said they stand united in supporting a return to the six-party talks which aimed to give North Korea economic aid and other concessions in exchange for abandoning its nuclear program. The talks also involve China, the U.S. and Russia.

The talks started in 2003 and made halting progress until April, when North Korea announced it would no longer participate.

Lee said it was necessary for the five negotiating parties to work together and exchange information on how to bring the North back to the bargaining table.

But the North has recently ramped up its strident , reiterating Sunday that it would bolster its nuclear arsenal as it accused the U.S. of plotting to launch a nuclear strike in a commentary carried by the state news agency.

Robert Dujarric, who directs the Japanese Studies program at Temple University's Tokyo campus, said the joint stance by Japan and South Korea on the North Korean issue was expected, as it echoes what they have said in the past and the countries are largely following the lead of the U.S. in negotiations.

He added that closer economic ties between the north Asian powers, especially given the current downturn, make sense, especially for Seoul.

"South Korea built its economy on the Japanese model, so it faces exactly the same challenges," he said.

Even before the financial downturn, Japan and South Korea had gradually been increasing their economic ties.

After previous free trade negotiations broke down in 2004 over trade barriers on agricultural and fishing products, they agreed to hold yearly meetings between their finance ministers. In December, Japan agreed to a currency swap deal of up to $30 billion with South Korea to help ease fears of a foreign exchange crisis there.