An alert has been raised after the largest active volcano in Japan began spewing plumes of smoke and ash nearly seven miles into the sky.

The Japanese Meteorological Agency issued its third highest alert after Mount Aso exploded in the early hours of Saturday morning, ejecting volcanic rock and ash 6.8 miles high. It also warned of heavy ash fall within 16 miles north east of the mountain.

Footage on Japan's NHK public television showed orange flames on several locations on the mountain top.

Cars are covered with volcanic ashes from Mount Aso in Aso (Reuters)

Police and municipal governments said there were no reports of injuries from the eruption, which began on one of the peaks of the 1,592m mountain in Kumamoto Prefecture - a popular hiking spot with tourists.

There are no homes within the off-limit area around the volcano, however nearby towns reported that buildings and cars had been covered in ash.

Aso city, about six miles north of the volcano, set up evacuation centres as a precaution and residents were pictured washing off ash from external walls, plants and streets.

Local media reported that some ash falls were detected in parts of western Japan.

A window at a youth centre located just a few miles away from the mountain was cracked, apparently by volcanic rocks. Masaaki Yamamoto, a manager at the center, told NHK he heard small volcanic rocks hitting the exterior of the building, and found a crack in the window along with chunks of volcanic debris around the size of a golf ball near it.

An aerial view shows an eruption of Mount Aso in Aso, Kumamoto (Reuters)

Up to 29,000 households lost power shortly after the eruption but the problem was fixed in less than two hours, a spokesman at Kyushu Electric told AFP.

The area experienced a number of earthquakes in April, however the agency said it was not known if the recent activity was related to the quakes. They said Mount Aso is unstable and could erupt again.

Mount Aso also erupted in September last year, blasting a plume of smoke 1.2 miles into the air.

In 2014, Mount Ontake in central Japan erupted unexpectedly, killing 63 people in the worst volcanic disaster in the country for nearly nine decades.