Japanese Prime Minister Fumio Kishida warned Saturday that Russia using nuclear weapons would be seen as an "act of hostility against humanity", describing President Vladimir Putin's saber-rattling as "deeply disturbing".

"Russia's act of threatening the use of nuclear weapons is a serious threat to the peace and security of the international community and absolutely unacceptable," said Kishida, who leads the only country ever hit with a nuclear bomb.

In May next year, Kishida is expected to host leaders from the G7 countries in Hiroshima, where a US nuclear bomb was dropped on 6 August 1945, resulting in the deaths of 140,000 people.

The Japanese city of Nagasaki was hit three days later.

Speaking in Australia, Kishida said the 77-year period of no nuclear weapons use "must not be ended".

"If nuclear weapons were ever used, that would be an act of hostility against humanity… the international community will never allow such an act," he said.

Since Russia invaded Ukraine in February, President Putin has made several thinly veiled threats about his willingness to deploy tactical nuclear weapons.