Japan's main ruling party has elected Chief Cabinet Secretary Suga Yoshihide its new leader.

The 71-year-old politician is virtually assured to succeed Prime Minister Abe Shinzo, who announced last month his intention to step down to deal with his health.

Two of Japan's biggest opposition groups are merging to challenge the ruling Liberal Democratic Party and gain control of the Diet.

The alliance on Thursday voted as its leader Edano Yukio, the head of one of the groups, the Constitutional Democratic Party of Japan. Edano said "if the LDP tries to dissolve the Lower House to hold an election and escape debating us in the Diet, we'll be ready to take them on and provide an alternative for the people."