The focus of the quake was 120 km (75 miles) below the surface of the earth in Iwate prefecture, a mountainous, sparsely populated region, the agency said.

"It was shaking so much that I almost couldn't step out of the kitchen and I panicked quite a bit. A lot of dishes broke," a man in Hachinohe city in Aomori prefecture, about 550 km northeast of Tokyo, told national broadcaster NHK.

Japanese media said military planes were flying over the area to try to assess the extent of damage and that local authorities had requested troops be sent to the area to help. Defence Ministry officials could not immediately be reached for comment.

"I don't have concrete information, but we've heard that there are several injured," Shinya Izumi, minister in charge of disaster management, told a news conference after the government set up an emergency task force at the prime minister's office.

Private broadcaster TV Asahi quoted a spokesman for a hospital in Hachinohe, a city with a population of about 240,000 some 550 km northeast of Tokyo, as saying that 13 people had been brought in with injuries, but gave no details on their condition.

Broadcaster TBS said 55 had been injured, while NHK put the figure at 18.

There was a fire in one building in the area after the quake, and NHK's fixed cameras showed fire engines driving through the streets towards the scene of the blaze, which it reported was soon put out.

Some parts of highways had been closed to traffic and some rail lines were stopped after the quake in the region, which is a mountainous and sparsely populated part of Japan, NHK reported.

"First it shook a little, then a strong shaking came. It shook for quite a long time," a civil servant in Iwate told NHK.

"Things didn't fall off the shelves. I saw some houses with shattered glass," he said.

Tohoku Electric said its nuclear facilities in the area were operating normally after the quake, except for one unit that was already off-line for maintenance work.

Tokyo Electric said its nuclear plants further south had not been affected.

Nippon Oil said its 145,000 barrels per day Sendai refinery was operating normally after quake, but Tohoku Electric said it had manually shut down a 250-megawatt oil-fired power plant in aomori after the quake.

Earthquakes are common in Japan, one of the world's most seismically active areas. The country accounts for about 20 percent of the world's earthquakes of magnitude 6 or greater.

Thursday's quake follows a string of earthquakes in the same region, the first of which in mid-June killed at least 10 people and left as many again missing.

In October 2004, an earthquake with a magnitude of 6.8 struck the Niigata region in northern Japan, killing 65 people and injuring more than 3,000.

That was the deadliest quake since a magnitude 7.3 tremor hit the city of Kobe in 1995, killing more than 6,400. (Reporting by Linda Sieg and Yoko Kubota; Additional reporting by Isabel Reynolds and Osamu Tsukimori; Writing by Chris Gallagher; Editing by Hugh Lawson)