Japan's Prime Minister Shigeru Ishiba has announced he is stepping down after less than a year in the role, following two major election losses.

The move comes a day before his Liberal Democratic Party (LDP) was expected to vote on whether to hold an internal leadership vote that could have forced him out.

The LDP has governed Japan for almost seven decades, but under Ishiba it lost its majority in the lower house for the first time in 15 years and then lost its majority in the upper house in July.

Japan, the world's fourth-largest economy and a key US ally, now faces a period of political uncertainty as tensions rise with China and regional insecurity mounts.

"Now that a conclusion has been reached in the negotiations concerning the US tariff measures, I believe this is precisely the appropriate time", Ishiba said, referring to a deal signed last week to ease tariffs imposed by US president Donald Trump on Japanese cars and other exports.