A two-year-old boy was rescued Wednesday after surviving four days buried in a landslide from the powerful quake that ravaged northern Japan over the weekend. Rescuers worked furiously to dig out his mother and sister.

The condition of the mother, Takako Minagawa, was uncertain. Workers had heard her voice earlier, but officials later said she appeared to have no pulse. Officials said Mayu Minagawa, 3, was alive earlier in the day, but that her condition also was unknown.

The family's white van was swept away in a wave of boulders and earth that pulverized the hillside road they were on when a 6.8-magnitude quake ripped across rural Niigata prefecture (search) on Saturday, killing at least 31 people.

"We got information from the site that they were able to locate (Takako Minagawa's) wrist, and it appears there are no signs of a pulse or other vital signs," said Home Affairs Ministry spokesman Koki Sasaki.

Firefighters dispatched from Tokyo (search) dug steadily through the rocks and rubble Wednesday afternoon after hearing Minagawa's voice in response to their calls. The van had been spotted under hillside rubble on Tuesday.

Officials were initially jubilant over the discovery.

"It's like a miracle that they are still alive after four days buried under a serious landslide like that, with huge rocks falling down on them," said Mikio Kawai, a spokesman for the Niigata prefectural government.

TV footage showed the son — Yuta, 2 — being pulled from the wreckage by the rescuers, covered in mud after four days under the landslide.

Some officials said both children had been pulled from the wreckage, but later said only Yuta was freed. The boy was flown by helicopter to hospital, said Keiko Kondo, a spokeswoman for Tokyo Fire Department.

The family was the focus of intense interest in Japan, with TV stations showing Minagawa's father's desperate attempts to track down the family in the post-quake chaos. The woman's husband was on assignment in Tokyo at the time of the quake.

The surprise rescue came hours after a powerful 6.1-magnitude quake rocked the region Wednesday morning, rattling already-damaged buildings and terrifying residents. At least five people were injured, and some buildings were damaged.

Homeless people in evacuation centers threw themselves to the floor and screamed in horror as the Wednesday temblor rumbled through the rural area of Niigata prefecture. A 4.2-magnitude aftershock hit about 25 minutes later.

The Japan Meteorological Agency (search) said the stronger quake was centered at a depth of 6 miles in Hirokami, a village of 9,200 about 150 miles northwest of Tokyo. The agency warned another quake of similar strength could hit in the coming hours or days.

Five people were injured in the Wednesday morning quake, but it was unclear if any of them were in serious condition. One man suffered a stroke from the shock of the quake, said Hisao Ishikawa of the Niigata prefectural government.

"Aftershocks are continuing — we don't know what the damage situation is yet," said Kazumasa Sakurai, an official at the Hirokami city hall.

The train station in Nagaoka, the largest city in the quake zone, was badly shaken and in danger of collapse, and hundreds of travelers on platforms and passengers on trains were ordered to evacuate, said an official at East Japan Railway Co.'s Niigata office.

The Meteorological Agency said there was no danger of tsunami waves.

The quake, which hit at 10:40 a.m., was so strong that people in temporary shelters had difficulty standing, national broadcaster NHK reported. The temblor swayed buildings in the Japanese capital.

By Wednesday morning, more than 440 aftershocks strong enough to be felt had hit since Saturday.