Japan's Nuclear Regulation Authority said that the Tomari nuclear power plant operated by Hokkaido Electric Power Co.'s, had lost an external power source, and a spent fuel pool was currently being cooled by an emergency power supply system.

The nuclear watchdog said that no abnormalities had been observed in radiation levels around the plant.

The JMA said there might be a slight sea-level change in Japan's coastal areas as a result of the early morning temblor.

A potentially lethal fire broke out at a petrochemical complex in Muroran, local media reported, and was later extinguished by firefighters.

Local police said they had received multiple reports of injuries as a result of the quake, including that of an 82-year-old man who was found with no vital signs after falling down the stairs. He was later found to have recovered.

In Sapporo, the capital city of Hokkaido, scores of people were reported injured, according to local firefighters, and a number of houses in the quake-stricken areas have collapsed, with emergency work underway to rescue those potentially buried under the houses, local officials said.

In Atsuma, at least 16 people were missing after 8 households were buried by landslides as of 8:00 am local time (2300 GMT Wednesday), according to public broadcaster NHK.