Troops have been called in after a second, more powerful earthquake hit southern Japan, killing 32 people, toppling large buildings and causing a massive landslide just over a day after an earlier deadly tremor.

Saturday's earthquake rocked Japan's south after a 6.2 quake hit near Mashiki town on Thursday, killing nine people and injuring about 1,000 others.

More than 1,500 people have been injured, 80 of them seriously, by the two quakes on the southwestern Kyushu island, Yoshihide Suga, Japanese government spokesperson, said.

Suga said the military would be boosted to 20,000 for rescue efforts. Police and firefighters were also being ordered to the southwestern region.

Japanese media also reported the eruption of Mount Aso, the largest active volcano in Japan located on the island. That was the first eruption in a month.

Smoke was rising about 100 metres but no damage has been reported.

The powerful earthquake on Saturday set off a huge landslide that swept away homes and cut off a highway in one area, and unlike the earlier quake which mostly affected old houses, larger buildings were damaged and some toppled across Kumamoto prefecture.

"There is a great possibly that the damage will spread widely so we must give it our all to gather the information on the damage situation and make the rescues and relief," Shinzo Abe, Japan's prime minister, said on Saturday.

After Thursday's initial tremors, more than 3,000 troops, police, and firefighters were dispatched to the area from around Japan.

About 44,000 people stayed in shelters.

Japan is frequently hit by major quakes. In March 2011, a 9.0-magnitude earthquake led to a devastating tsunami that killed 18,000 people along Japan's northeast coast.

The wave struck the Fukushima nuclear plant, causing a major radiation leakage.

More than 100,000 displaced people are still unable to return to their homes near the nuclear plant because of the contamination.

The city office in Uto was badly damaged and said to be in danger of collapse, while aerial footage shot by broadcaster TBS showed the toppled centuries-old Aso shrine, its main gates flattened and wooden columns reduced to rubble.

Airport closed

In Kumamoto city, authorities evacuated patients from a hospital over fears it could collapse and images showed the tilted building.

The region's transport network suffered considerable damage with one tunnel caved in, a highway bridge damaged, roads blocked by landslips and train services halted, media reported.

Kumamoto airport was forced to close after a ceiling collapsed from the shaking, Jiji Press reported, with no immediate plans to resume flights, and communications in the area were spotty.

Gen Aoki, a Japan Meteorological Agency official, said Saturday's quake was the strongest to hit in recent days, and that Thursday's was merely a "precursor".

The US Geological Survey measured the quake at magnitude 7.0, or 6.3 times bigger than the 6.2 tremor recorded on Thursday.

There have been more than 230 aftershocks of at least level 1 on the Japanese scale since Thursday's shock, said Japan's meteorological agency.

Japan is on the seismically active Ring of Fire around the Pacific Ocean and has building codes aimed at helping structures withstand earthquakes.