TOKYO >> A powerful earthquake jolted northwestern Japan today, and officials warned of a possible tsunami up to 3.3 feet high along parts of the coast.

Some areas lost electrical power and some bullet train services were suspended, but there were no immediate reports of serious injuries or damage from the magnitude 6.8 quake. Only a minor swelling of the sea was observed in several cities about half an hour after it struck.

“Based on all available data a destructive Pacific-wide tsunami is not expected and there is no tsunami threat to Hawaii,” the Pacific Tsunami Warning Center said in a bulletin.

Japan’s Meteorological Agency said the earthquake was located off the western coast of Yamagata about 30 miles southwest of the city of Sakata.

It said the quake was fairly shallow, about 6 miles below the sea’s surface. Shallow quakes tend to cause more damage on the Earth’s surface.

The agency warned of a possible tsunami along the coasts of the northwestern prefectures of Yamagata, Niigata and Ishikawa.

Bullet train service was suspended in parts of the region because of power outages and for safety checks. About 9,000 households in Yamagata and Niigata lost power, according to Tohoku Electric Power Co.

All seven reactors at the Kashiwazaki-Kariwa nuclear plant in Niigata were offline and no abnormalities were reported.

Chief Cabinet Secretary Yoshihide Suga said an emergency response team was set up at the Prime Minister’s Office to assess the extent of injuries and damage. He urged residents of the affected areas to be prepared for possible aftershocks.

An elderly man suffered a minor foot injury in Murakami city, where strong shaking was reported, public broadcaster NHK said.

NHK showed broken glasses and dishes scattered on the floor of a bar in Tsuruoka which was empty after customers rushed out, leaving behind their half-eaten food on the counter.

Japan is one of the most earthquake- and tsunami-prone areas in the world. On March 11, 2011, a magnitude 9.0 offshore quake hit the northeast coast, causing a tsunami that took more than 18,000 lives and triggered meltdowns at the Fukushima nuclear power plant.

There was no danger of a tsunami from Tuesday’s quake on the eastern coast, where the Fukushima plant is located.