Japan could be nearly destroyed by a massive volcanic eruption over the next century, putting almost all of the country's 127 million-strong population at risk, according to a new study.

"It is not an overstatement to say that a colossal volcanic eruption would leave Japan extinct as a country," Kobe University earth sciences professor Yoshiyuki Tatsumi and associate professor Keiko Suzuki said in a study publicly released on Wednesday.

The experts said they analysed the scale and frequency of volcanic eruptions in the archipelago nation over the past 120,000 years and calculated that the odds of a devastating eruption at about one percent over the next 100 years.

The chance of a major earthquake striking the city of Kobe within 30 years was estimated at about one percent just a day before a 7.2-magnitude quake destroyed the Japanese port city in 1995, killing 6,400 people and injuring nearly 4,400 others, the study noted.

"Therefore, it would be no surprise if such a colossal eruption occurs at any moment," it added.

The new research comes weeks after Japan's Mount Ontake erupted without warning -- killing 57 people and leaving at least six others missing in the country's deadliest volcanic eruption in almost 90 years.

The Kobe University researchers said their study was critical because Japan is home to about seven percent of the volcanoes that have erupted over the past 10,000 years.

A disaster on the southernmost main island of Kyushu, which has been struck by seven massive eruptions over the past 120,000 years, would see an area with seven million people buried by flows of lava and molten rock in just two hours, they said.

Volcanic ash would also be carried by westerly winds toward the main island of Honshu, making almost all of the country "unliveable" as it strangled infrastructure, including key transport systems, they said.

It would be "hopeless" trying to save about 120 million living in major cities and towns across Honshu, the study said.

This prediction was based on geological findings from the eruption of a gigantic crater, 23 kilometres (14 miles) across, in southern Kyushu about 28,000 years ago.

The study called for new technology to accurately grasp the state of "magma reservoirs" which are spread across the earth's crust in layers a few kilometres deep.