The outgoing finance minister promised a "new start" after ruling Democratic Party (DPJ) members chose him as their new leader.

He becomes Japan's fifth prime minister in three years.

In a speech to DPJ members on June 4, 63-year-old Kan said the alliance with the US would remain the "cornerstone" of Japan's diplomacy, but he made no pledges about the base.

"Prime Minister Hatoyama opened the door for a new DPJ to be built," Mr Kan was quoted as saying by the Yomiuri Shimbun newspaper.

"We have to rebuild the party and make a brand new start."