TOKYO - Japan's opposition leader, a strong contender to become the next prime minister, said Monday that Japan's ties with the United States are biased too heavily toward defence and said the focus should be shifted to the economy.

Yukio Hatoyama, the leader of the Democratic Party of Japan, made the comments at a debate among the leaders of six major political parties at the Japan National Press Club kicking off campaigns for parliamentary elections to be held Aug. 30.

Hatoyama's party is expected to make big gains. If it does, he could become Japan's next prime minister.

In the debate, he promised to stick to the country's pacifist principles and said he would end a refuelling mission in the Indian Ocean in support of U.S.-led operations in Afghanistan. If not extended, the mission will end in January.

"Japan's relations with the U.S. have been heavily biased toward defence," Hatoyama said. "Now it's time to shift our focus to economic ties. We will strengthen our economic ties and promote free trade while protecting our national interests."

Hatoyama said he supports President Barack Obama's commitment to a nuclear free world. He added more effort should be put into diplomacy than arms.

Prime Minister Taro Aso, meanwhile, focused on recent economic gains as the reason why voters should keep his party in power.

Aso's Liberal Democratic Party has governed for virtually all of the past six decades, but is on the verge of defeat in the upcoming elections for the lower house of parliament, according to polls.

"The LDP is the one party that has policies that are consistent and has the ability to achieve them," Aso said.

He cited recent signs of economic recovery as proof a series of stimulus measures under his leadership has taken effect.

The Cabinet Office said earlier Monday that the world's second-largest economy grew at an annual rate of 3.7 per cent in the April-June quarter, rising for the first time in more than a year.