TOKYO - Former foreign minister Seiji Maehara's chances of becoming Japan's next prime minister were clouded on Friday after domestic media said party powerbroker Ichiro Ozawa was unlikely to back the 49-year-old security hawk's bid for the post.

Japan's Prime Minister Naoto Kan leaves a memorial ceremony marking the 66th anniversary of Japan's surrender in World War Two at Budokan Hall in Tokyo August 15, 2011. [Photo/Agencies]

Prime Minister Naoto Kan, under fire for his response to the massive March tsunami and the nuclear crisis it triggered, was expected later on Friday to confirm his intention to step down, clearing the way for Japan to pick its sixth leader in five years.

Whoever takes over as Japan's next leader faces a mountain of challenges, including a strong yen, rebuilding from the devastation of the March disasters, ending the radiation crisis at a crippled nuclear plant and forging a new energy policy, and curbing massive public debt while funding the bulging social security costs of a fast-ageing society.

It was unclear whether Ozawa, who heads the ruling Democratic Party of Japan's (DPJ) biggest group despite suspension of his membership over a funding scandal, would back an existing candidate such as trade minister Banri Kaieda or try to find a new contender to support, the Yomiuri newspaper said.

"We definitely cannot trust Mr. Maehara," the paper quoted a source close to Ozawa as saying.

Maehara, who is promising to focus economic policy on promoting growth and beating deflation, is the most popular with voters among seven contenders jostling for the nation's top job, but only DPJ lawmakers can vote in the Aug 29 party leadership race. The winner will become premier because of the party's majority in parliament's lower house.

Rifts over the role of Ozawa, a political mastermind saddled with an image as an old-style wheeler-dealer, have plagued the Democratic Party ever since it swept to power in 2009 promising change.

The party's voter support has sunk since as it struggled to implement policies in the face of internal feuds and a divided parliament, where the opposition can block bills.