AFTERSHOCKS CONTINUING
"The aftershocks are continuing ... so a very careful response is required," Shinya Izumi, the cabinet minister in charge of disaster response, told a news conference.
"But we also need to rescue people as quickly as possible. It is a very tough situation."
Two of three people missing at a work site in Kurihara after a landslide had been found, and were in cardiac arrest, NHK said. Four campers including three non-Japanese, were unreachable, Kyodo news agency said.
More than 600 people were cut off in remote areas and military and other helicopters were heading their way, NHK said.
"I don't know if my family is safe," one middle-aged man covered in mud to the knees after hiking to a hot spring resort to check on relatives, told Reuters at an elementary school where the military had set up a landing pad.
Experts said casualties could rise as reports came in from isolated areas but the scope of the quake was far smaller than one that stuck China a month ago.
"The seismic energy of the China quake was one order of magnitude greater," Naoshi Hirata, a professor at Tokyo University's Earthquake Research Institute, told Reuters.
He added the region's sparse population and Japan's strict building standards had likely limited the impact.
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, prompting tough building codes to try to limit damage.
In October 2004, a magnitude 6.8 earthquake 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.
