A strong 6.4-magnitude earthquake struck off northeast Japan's Fukushima region, home to a crippled nuclear power plant, early Sunday, but there was no risk of a tsunami, seismologists said.

The tremor at 3:54 am (1854 GMT Saturday) injured seven people but caused no damage to the Fukushima Daiichi plant, which was ravaged by a 9.0 quake and ensuing tsunami in March, the national Fire and Disaster Management Agency said.

In the city of Hitachinaka, a 60-year-old man broke his wrist when he was jolted off his bed by the tremor, media reports said. Two elderly women, aged 69 and 90, were hurt when they fell over at their homes in Koriyama.

The Japan Meteorological Agency said the tremor was presumed to be an aftershock of the March 11 quake.

"Aftershocks of the massive earthquake are still continuing actively," Akira Nagai, director of earthquake and tsunami monitoring at the agency, told a news conference.

The quake was centred in the Pacific around 100 kilometres (60 miles) south-southeast of Fukushima city, the agency and the US Geological Survey said, at a depth of about 40 kilometres.

The March disaster left more than 20,000 people dead or missing on the country's northeast coast, and the damaged nuclear plant leaking radioactive substances from its reactors.

Japan, located at the junction of four tectonic plates, experiences 20 percent of the strongest quakes recorded on Earth each year.