A powerful earthquake has struck the Japanese island of Hokkaido, killing at least nine people and leaving about 30 missing.

Key points: About 30 people are missing, nine are confirmed dead

About 30 people are missing, nine are confirmed dead Nearly 3 million households without power

Nearly 3 million households without power Nuclear plant suffers power outage

Many are trapped in landslides triggered by the magnitude-6.7 quake, and electricity has been cut to 3 million homes.

The US Geological Survey said the earthquake struck about 68 kilometres south-east of Sapporo, Hokkaido's main city.

Authorities are trying to reach those worst affected by air, deploying 51 helicopters and 25,000 Self Defence Force officers as part of the rescue effort.

At its strongest, the quake registered at more than six on the shindo intensity scale, meaning it is impossible to remain standing or move without crawling.

Japan's northernmost island is best known to Australians for its powder snow and the Niseko ski resort — but that steep terrain makes it prone to landslides.

The recent typhoon which swept through Japan also brought heavy rains to the area, loosening the terrain.

'The mountain slid down'

In the town of Atsuma, where entire hillsides collapsed, rescuers used small backhoes and shovels to search for survivors under the tons of earth that tumbled down steep mountainsides, burying houses and farm buildings below.

The area's deep green hills were marred by reddish-brown gashes where the soil tore loose under the violent tremors.

Twenty-eight people remained unaccounted for in the town, Atsuma Mayor Shoichiro Miyasaka told public broadcaster NHK.

"We will carry on searching for them," he said.

The earthquake left roads damaged, and caused landslides that crushed homes. ( Hiroki Yamauchi/Kyodo News via AP )

The quake struck at 3:08am (local time) when most people were asleep — and stories are starting to emerge about the moment it hit.

"The mountain slid down, I had no idea what had happened," a woman at an evacuation centre said.

"When it became brighter I was surprised to see the view — I couldn't believe it.

"Something unbelievable was happening."

Another woman said she was in shock.

"I have tears in my eyes," she said.

"It's completely different from the usual view and I never thought such a thing would happen inside my home."

Smoke billows from the facilities of Mitsubishi Steel Muroran during a fire. ( Hokkaido Shimbun/Kyodo News via AP )

Millions without power

Hokkaido is facing a power crisis caused by a fire inside the Tomato-Atsuma coal thermal power plant.

A fire broke out inside a turbine and the plant also suffered damage to a boiler pipe, according to Japan's Trade and Industry Minister, Hiroshige Seko.

"Their biggest thermal power plant in Hokkaido was working to resume but they detected fire," Mr Seko said.

"The boilers are damaged at number one and number two plant.

"The fire was at the number four plant, near the turbine."

It is not clear if the quake caused the fire.

Sorry, this video has expired Landslides damaged houses and millions were left without power after the earthquake hit (Image: Masanori Takei/Kyodo News via AP)

There are more than 2.95 million people without power on the island and it is expected to take a week to repair the plant.

Authorities are relying on hydro-electric plants and are bringing generators to the island, according to Mr Seko.

The nearby Tomari nuclear power plant remains inactive — and has been since the March 2011 quake and tsunami.

However, power has been cut to the plant and it is relying on emergency generators.

It's understood they can last for a week.

Earthquakes, typhoons, floods

A road damaged by the earthquake in Sapporo, Hokkaido. ( Hiroki Yamauchi/Kyodo News via AP )

A fire broke out at a Mitsubishi Steel plant in the city of Muroran after the quake but was mostly extinguished with no injuries, a company official said.

A row of houses could be seen slanting at odd angles, leaning against one another in one town, and many schools were closed, NHK said.

A series of smaller shocks, including one with a magnitude of 5.4, followed the initial quake, the Meteorological Agency said.

Agency official Toshiyuki Matsumori warned residents to take precautions for potential major aftershocks in coming days.

Sorry, this video has expired The quake comes days after Typhoon Jebi battered western Japan.

Meanwhile, roof tiles and water covered floors at Hokkaido's New Chitose Airport, which would be closed for at least the rest of the day.

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Kyodo news agency said more than 200 flights and 40,000 passengers would be affected on Thursday alone.

The closure came just days after Kansai Airport, another major regional hub in western Japan, was shut by Typhoon Jebi, which killed 11 people and injured hundreds.

The storm, the most powerful to hit Japan in 25 years, stranded thousands of passengers and workers at the airport, whose operator said it would resume some domestic flights on Friday.

Prime Minister Shinzo Abe said his Government had set up a command centre to coordinate relief and rescue and said saving lives was the top priority.

ABC/Reuters