A strong 6.2 magnitude earthquake has struck central Japan, causing houses to collapse and injuring at least 20 people, three seriously.

The quake struck at 10.08pm local time on Saturday at a depth of 10km at the epicentre, in the north of Nagano Prefecture, northwest of Tokyo, according to the US Geological Survey.

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Television footage showed flattened wooden houses, a mudslide blocking a road and books toppled from shelves.

Twenty-one people were trapped under debris but have since been rescued, Kyodo news agency said, citing police.

The Japan Meteorological Agency measured the quake at magnitude 6.8. No tsunami alert was issued.

“It’s quite a strong earthquake for an inland one,” an official at the Japanese agency told a midnight press conference.

“We are worried about the extent of damage to houses and buildings,” he said, adding that residents should also be on alert for possible landslides as mountainous areas were the hardest hit.

The quake toppled at least five houses in Hakuba Village, Nagano Prefecture, local police said, injuring two residents.

According to reports on public broadcaster NHK, 21 residents were temporarily trapped under the houses but all of them, including the injured people and a two-year-old toddler, were rescued.

Electricity was cut off in around 1600 households.

Nagano police said several people sustained minor injuries in the quake.

More than 20 aftershocks hit Nagano following the first big strike.

There was no damage to any of the seven nuclear reactors at the sprawling Kashiwazaki-Kariwa plant in neighbouring Niigata prefecture as they have been offline since 2011.

A strong tremor revives memories of the 9.0 earthquake in March 2011, which triggered a tsunami that sparked the Fukushima atomic plant disaster and left 18,000 people dead or missing.