TOKYO (Reuters) - Japan will compile a considerably large supplementary budget immediately after the upcoming general election to ease pandemic pain and back long-term growth in areas such as green, digital and infrastructure, a ruling party heavyweight said on Sunday.

       "What must be tackled at first is vaccinations. This is the strongest of coronavirus measures," Akira Amari, newly appointed secretary-general of the Liberal Democratic Party (LDP), told public broadcaster NHK's political debate programme.

       Japan should bring the vaccination rate from around 60% at present to as much as 70% to 80%, levels seen as easing anxiety among the public, Amari said.

       "We have responded with various measures by tapping emergency budget reserves. Now the reserves are drying up, we will compile a considerably large extra budget immediately after election."

       Given dire public finances, Japan's next prime minister, Fumio Kishida, may have little choice but to sell more government bonds to fund a pandemic-relief package that he said would be worth hundreds of billions of dollars.

       Kishida, a former foreign minister, won the LDP's leadership race on Wednesday and is almost certain to take over Yoshihide Suga as premier on Monday by virtue of the party's majority in the powerful lower house.

       Kishida is widely expected to be officially voted in as prime minister when parliament is convened on Monday and will announce a cabinet reshuffle later in the day.

       Amari, a former economy minister seen as a key ally of Japan's longest-serving premier Shinzo Abe, was appointed secretary-general on Friday.

       (Reporting by Tetsushi Kajimoto; Editing by Christopher Cushing)