TOKYO -- Mount Aso, a volcano on the southern Japanese island of Kyushu, has erupted, sending huge plumes of black and then white smoke 2,000 metres into the air.

The eruption Monday prompted authorities to raise the alert level for the area, banning people from approaching within 2 kilometres of the volcano's mouth.

There are no homes within that area and there were no immediate reports of injuries or damage.

The national television network NHK cited local police as saying they had safety evacuated people from a ropeway station on the mountain and were having other visitors move to lower elevations. Police were still checking to see if any hikers were in the restricted area.

Some flights to the nearby city of Kumamoto were diverted due to the ash pumped into the air by the eruption.

The Japan Meteorological Agency said the eruption fired bits of volcanic rock, smoke and steam into the air. It was unclear if there was any volcanic flow from the mountain.

"This could happen a few times," Yasuaki Sudo, a researcher at the Aso Volcano Museum, told NHK. "The most important thing is to stay out of the danger zone."

The Japanese archipelago sits atop the Pacific "ring of fire" and has more than 100 volcanoes. Mount Aso had a smaller eruption in August and eruptions in 2014 that disrupted tourism in the area.

During the summer, authorities closed part of a popular hot springs about 80 kilometres from Tokyo because of fears of an eruption of Mount Hakone, which is southeast of Mount Fuji.

The eruption last September of another volcano, Mount Ontake in central Japan, killed 57 people.