Rescuers search for missing persons at the site of a landslide triggered by Thursday's earthquake in Atsuma, Hokkaido, northern Japan Saturday, Sept. 8, 2018. (Kyodo News via AP)

Tokyo - Japanese rescuers were Saturday desperately searching for missing residents whose houses were buried by landslides triggered by a powerful earthquake on the northern island of Hokkaido.

At least 21 people died in the magnitude-6.7 quake that hit Hokkaido early Thursday. Thirteen people remain missing, government spokesman Yoshihide Suga told a news conference.

About 40,000 people, including soldiers, firefighters and police officials, "are working around the clock to engage in search and rescue operations," Suga said.

The search operations were focusing on a sparsely populated area of Atsuma Town, where the quake set off massive landslides that destroyed houses and roads.

About 12,000 people were taking shelter in temporary evacuation centres while a series of aftershocks continue to jolt the region.

Suga also warned of mudslides in disaster-stricken areas as heavy rain was forecast for Hokkaido over the weekend.

Power has been almost restored two days after the quake caused a blackout across the whole island, resulting in nearly 3 million households losing electricity and affecting businesses and hospitals. As of Saturday afternoon, about 10,600 households remained without power, Hokkaido Electric Power said.

Toyota Motor said it had decided to halt almost all its production in Japan on Monday as its factory on Hokkaido was closed due to the blackout. Japan's largest carmaker produces transmissions at the plant.