TOKYO: An earthquake with a preliminary magnitude of 6.7 rocked Japan's northernmost main island of Hokkaido early Thursday, cutting power supplies, disrupting public transportation and causing mudslides that buried houses, authorities said.

A number of houses were buried in the towns of Atsuma and Abira following the 3:08am (1.08am Thailand time) quake after mountain slopes collapsed, according to the prefectural government and firefighters.

In Atsuma, two people were confirmed killed by the quake and 39 people were missing.

An 82-year-old man was found to be showing no signs of life after falling down the stairs in his residence in Tomakomai but later recovered, police said.

The Nuclear Regulation Authority said Hokkaido Electric Power Co's Tomari nuclear power plant lost an external power source, with a spent fuel pool at its Nos 1 to 3 reactors currently being cooled by an emergency power supply system.

No abnormality has been confirmed in radiation levels around the plant, regulators said. Meanwhile, Tohoku Electric Power Co said no abnormalities were detected at the Higashidori nuclear power plant in Aomori Prefecture.

Around 3 million houses were left without power in Hokkaido after the earthquake caused thermal power plants in the prefecture to shut down. The blackout affected telephone service and television broadcasting in Sapporo.

A fire broke out at a petrochemical complex in Muroran and was later extinguished by firefighters.

The temblor, which occurred in southern Hokkaido at a depth of about 37 kilometres, logged upper 6 on the Japanese seismic intensity scale of 7 in the town of Abira and lower 6 in the city of Chitose, both southeast of Sapporo. It was followed by smaller aftershocks. No tsunami warning was issued.

It was the first time for a quake in Hokkaido to reach an intensity of upper 6 since the seismic scale was revised in 1996.

"It messed up my entire house. I've never experienced an earthquake like this," an 87-year-old man in Atsuma said.

New Chitose Airport will be closed throughout the day after part of its ceiling collapsed as well as due to the power outage, the transport ministry said, affecting flights from Tokyo and other cities.

Bullet train and local train services were also disrupted by the quake, operators said.

In Tokyo, the central government said the Self-Defence Forces will dispatch 25,000 personnel for relief operations at the request of the governor of Hokkaido.