Scores of people were feared buried alive Saturday after two powerful quakes hit southern Japan a day apart, killing at least 35 people, as a forecast storm threatened more devastating landslides.

Homes, roads and railway lines were swept away when huge hillsides collapsed as thousands of tonnes of mud was dislodged by the thunderous seismic tremors.

Buildings were reduced to rubble, including a university dormitory and apartment complexes, with dozens of people unaccounted for over a wide area.

"We are aware of multiple locations where people have been buried alive," Chief Cabinet Secretary Yoshihide Suga told a press conference.

"Police, firefighters and Self Defense Force (military) personnel are doing all they can to rescue them."

More than 90,000 people have been evacuated, including 300 from an area close to a dam thought to be at risk of collapse.

A hospital was left teetering by Saturday morning's 7.0 quake, with doctors and patients rushed from the building in darkness.

Isolated villages in mountainous areas near Kumamoto city were completely cut off by landslides and damage to roads.

At least 1,000 people were believed trapped in one settlement and expected to spend the night in public buildings, reports said.

Aerial footage showed a bridge on a main trunk road had crashed onto the carriageway below it, its pillars felled.

The quake came as emergency responders were working to reach areas already hit by a 6.2 magnitude tremor that struck late Thursday.

Both quakes were very shallow and caused violent shaking.

Adding to worries was the eruption of a nearby volcano, although seismologists cautioned there was no evidence of a link and said activity was limited.

- AFTERSHOCKS -

Aftershocks continued to rock Kumamoto on Kyushu island and its surroundings, an area unaccustomed to the powerful quakes that regularly rattle other parts of Japan.

Thursday's initial quake affected older buildings and killed nine people, but Saturday's brought newer structures crashing down, including a municipal office in the city of Uto.

"The total number of deaths reached 35," Mariko Kuramitsu, a spokeswoman at the Kumamoto prefectural government, told AFP.

Nearly 1,000 people have been hurt, 184 of them seriously, she added.

Tokai University announced that two of its students, who were among around a dozen trapped in a dormitory building in Minami-Aso, were now known to have died.

"We offer our sincerest prayers for the two," said a statement on its website. "We're trying to confirm the safety of other students."

At least one of the dead was killed when a fire ripped through an apartment complex in the town of Yatsushiro, a local official said.

In nearby Kumamoto city, an AFP journalist said he was jolted awake by powerful shaking, which sent the television set in his hotel room crashing to the floor. Staff urged guests to evacuate.

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

By nightfall more than 100,000 households were still without electricity.

- 'REALLY STRONG' -

Japan's Meteorological Agency said rain was expected to hit Kumamoto overnight and some areas would see heavy downpours on Sunday, raising the risk of further landslides in places where soil and rock has already been loosened.

Twitter user @kbbblove pleaded for help for "about 30 people" still trapped at a campsite.

"Please go rescue them before the rain and wind comes because it's close to a mountain," the user wrote.

The town of Misato urged more than 10,000 people to evacuate Saturday for fear of a landslide, national broadcaster NHK reported.

Prime Minister Shinzo Abe announced he was sending 20,000 emergency responders to the area, including troops, firefighters and medics.

"From tonight, we are expecting rain and winds and deteriorating weather in the Kyushu region," he told a press conference.

"There are fears of possible landslides and other secondary disasters."

Japan, one of the world's most seismically active countries, suffered a massive undersea quake on March 11, 2011 that sent a tsunami barrelling into its northeast coast.

Some 18,500 people were left dead or missing, and several nuclear reactors went into meltdown at the Fukushima plant in the worst atomic accident in a generation.

Japan's only working nuclear plant, southwest of Saturday's epicentre, was unaffected by the quake, the government has said.