TOKYO, Jan 14 - A magnitude 6.7 earthquake struck just off Japan's northernmost main island of Hokkaido on Thursday, Japan's Meteorological Agency said.

No tsunami warning was issued and NHK national television said that while there might be some changes in sea level, no tsunami damage was expected.

There were no immediate reports of damage.

No irregularities were reported at Hokkaido Electric Power's Tomari nuclear plant and Tohoku Electric Power's Higashidori nuclear plant in northern Japan, both of which have been kept shut pending stringent safety checks following the Fukushima disaster, spokesmen at both firms said.

There were also no irregularities at the Rokkasho plutonium preprocessing and other nuclear-related facilities in Aomori prefecture, operated by Japan Nuclear Fuel Ltd, the Nuclear Regulation Authority said.

On March 11, 2011, the northeast coast was struck by a magnitude 9.0 earthquake, the strongest quake in Japan on record, causing a massive tsunami. Those events triggered the world's worst nuclear crisis since Chernobyl a quarter of a century earlier.