The damage from an earthquake in a mountainous area of central Japan is proving more extensive than initially thought.

A daylight assessment has found at least 50 homes destroyed in two villages, and 41 people injured across the region, including seven seriously, mostly with broken bones.

The Japan Meteorological Agency says the magnitude-6.7 earthquake struck shortly after 10pm Saturday west of Nagano at a depth of five kilometres."

Japan's Nuclear Regulation Authority said no abnormalities were reported at three nuclear power plants in the affected areas.