A powerful six-point-seven magnitude earthquake struck off the northwestern coast of Japan late Tuesday, injuring at least 15 people.

According to Japan's Meteorological Agency, the earthquake occurred in waters some 50 kilometers southwest of the city of Sakata at around 10:22 p.m. 

The weather agency issued a tsunami warning right after the quake, predicting a tsunami as high as one meter along the coasts of the northwestern prefectures of Yamagata, Niigata and Ishikawa. 

However, only a minor swelling of the sea was observed in some areas about two hours after the quake struck. 
 
Some areas lost electrical power and some train services were suspended for safety checks, but have since started running again.

Japan's atomic regulator said nuclear reactors in Niigata were off line and no abnormalities were reported from the quake.