According to public broadcaster NHK, officials have lost contact with 20 people near the epicenter and an 82-year-old man died after falling down the stairs at his home during the quake.
At central Hokkaido's scenic town of Biei, residents lined up outside of supermarkets, clearing shelves of water, toilet paper and food, reported Associated Press.
It had been hoped that power would be restored in hours and some electricity gradually returned but damage to generators at the plant means it may take more than a week before it is back up, Mr Seko said.
At least seven people have died and dozens are trapped in their homes after a powerful natural disaster hit the Japanese island of Hokkaido.
A magnitude 6.7 quake struck southern Hokkaido soon after 6am NZT, at a depth of 40 kilometres, Japan's Meteorological Agency said.
"While we have yet to confirm the full picture and magnitude of the damages suffered, there are reports on large-scale landslides and power outages, and the entire region is still under an unpredictable condition".
Prime Minister Shinzo Abe, arriving at his office before 6 a.m., told reporters his government had set up a command centre to coordinate relief and rescue.
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Hokkaido's Tomari nuclear power station, which has been shut since the 2011 quake and tsunami, suffered a power outage but officials said it was cooling its spent nuclear fuel safely.
Toyota Motor Corp. said it halted daytime operations at its subsidiary's transmission manufacturing plant in Tomakomai, southern Hokkaido, and made a decision to suspend nighttime operations as well, as it is unclear when power will return to normal.
Industry Minister Hiroshige Seko said the Hokkaido Electric Power Co. might need a week to restore power fully to all 5.3 million residents. "Due to the effects of the quake, Japan Football Association has made a decision to cancel the Kirin Challenge Cup 2018 match between Samurai Blue [Japan National Team] and Chile National Team, scheduled to be played at Sapporo Dome on 7 September 2018".
A strong quake that rocked Kumamoto Prefecture in 2016, killing 50 people, had a focal depth of 11 km and was followed by numerous aftershocks.
Houses destroyed by the landslide in Atsuma town.
The tsunami also damaged the Fukushima Daiichi nuclear power plant, leading to a series of explosions and meltdowns that spewed radiation into the air and ocean. Several people were reported missing in the nearby the town, where a massive landslide engulfed homes in an avalanche of soil, rocks and timber.
Residents watch a road damaged by an quake in Sapporo, Hokkaido, northern Japan.
A powerful 6.7-magnitude quake rocked the island of Hokkaido in Japan early today.