SENDAI, Japan (CNN) -- As Japan buckles from a powerful earthquake and a devastating tsunami, residents on Sunday hoped they are spared an even more catastrophic fate: a widespread release of radiation from damaged nuclear plants.
By Sunday afternoon, the death toll from the country's strongest earthquake in more than a century and the crushing walls of water that followed had risen to 985.
An additional 707 were missing, the National Police Agency said. Officials fear the numbers may climb once rescuers reach more hard-hit areas.
The nuclear plants sparked fresh concerns for survivors of an 8.9-magnitude earthquake that tore through Japan on Friday, triggering massive waves that ravaged everything on their path.
"I've not slept since Friday because of aftershocks," said Indri Rosid, who lives in Tokyo.
"Now I have nuclear plants to worry about. We have an idea of what to do when an earthquake hits, but what should I do in a radiation leak?"
Rosid said she has an earthquake emergency kit that includes a flashlight, documents and canned food.
"But I have none for a radiation leak because no one teaches you what to do in that case," she said.
Officials do not know for certain whether there have been meltdowns at two reactors in a nuclear facility in the northeast, said Yukio Edano, the Chief Cabinet Secretary.
They are working under the presumption that such meltdowns have taken place as they attempt to cool down radioactive material and release pressure inside the reactors at the Fukushima Daiichi plant, he said.
A meltdown is a catastrophic failure of the reactor core, with a potential for widespread radiation release.
So far, there have been no indications of dangerously high radiation levels in the atmosphere around the plant.
Later Sunday, he said a second explosion could occur at the plant.
An explosion caused by hydrogen buildup Saturday blew the roof off a concrete building housing the plant's No. 1 reactor, but the reactor and its containment system were not damaged in the explosion.
"There is a possibility that the third reactor may have hydrogen gas that is accumulating in the reactor (that) may potentially cause an explosion," he said.
Edano said the No. 3 reactor would also likely withstand such a blast, noting that workers had already released gas from the building to try to prevent an explosion.
Workers have been scrambling to cool off fuel rods at both reactors after the massive earthquake and tsunami disabled their cooling systems.
Later Sunday, a spokesman for Japan's prime minister said he would not describe what was occurring in the reactors as a "meltdown."
"The situation is under control. ....We have been succeeding in lowering pressure inside the containment vessel," spokesman Noriyuki Shikata said.
Still, the government evacuated more than 200,000 residents from homes close to the plant, and tested 160 people for radiation exposure on Sunday, authorities said.
"We have the cars filled up and ready for an emergency drive back home to Kyushu in case things get ugly," said Fulco Vrooland, referring to the most southwesterly of Japan's islands.
Meanwhile, in neighborhoods swallowed by tsunami-triggered walls of water, rescuers and shell-shocked residents scrambled to reach survivors.
In the city of Ishinomaki, the military was going door-to-door, hoping to find survivors. Instead, they mostly found bodies of elderly people.
In one coastal town alone -- Minami Sanriku, in Myagi Prefecture -- some 9,500 people, half the town's population, were unaccounted for.
Rescuers trudging through water-logged, debris-filled streets found the city of Sendai in ruins. Cars were stacked on top of one another; and a carpet of sludge covered the remains of what used to be homes.
Sendai lies 130 kilometers (80 miles) east of the earthquake epicenter
About 2.5 million households -- just over 4% of the total in Japan -- were without electricity Sunday, said Ichiro Fujisaki, the nation's U.S. ambassador. Lights were turned off in most landmarks to save energy, including the Tokyo Tower and Rainbow Bridge in Tokyo.
Scores lined up at the few gas stations, drug stores and supermarkets that were open. Shelves were largely empty as stores rushed to restock.
'We've been provided some water rations ... and we're still not sure when we are to get more," said Matthew Williams, who lives in Shin-Urayasu near Tokyo. "The city has told us we are able to take a bucket to the local elementary school to obtain some water, but the wait is about three hours."
Residents also braved an seemingly endless barrage of aftershocks.
"The aftershocks still keep coming every 10 minutes and my house sways every time," said Tokyo resident Shintaro Higuchi on Sunday, two days after the 8.9-magnitude struck 373 kilometers (231) miles away.
Japanese officials raised the quake's magnitude to 9.0 on Sunday. But the U.S. Geological Survey kept its magnitude at 8.9.
The worse may not be over. There's a high chance of a magnitude-7.0 quake or above in the next three days because of increased tectonic activity, the earthquake prediction department chief for the Japan Meteorological Agency said Sunday.
The Japanese agency canceled all tsunami warnings Sunday, but said more warnings and advisories are likely to be issued because of aftershocks.
The USGS reported scores of such aftershocks. More than two dozen were greater than magnitude 6, the size of the quake that severely damaged Christchurch, New Zealand last month, the agency said.
And the death toll is expected to surge. The number of dead in Miyagi prefecture alone "will undoubtedly be in the tens of thousands," Japanese public broadcaster NHK reported, citing the prefecture's police chief.
Japan plans to dispatch 100,000 members of its defense forces to the quake-ravaged region -- double the previous number -- authorities said Sunday.
"We are extending emergency food, drinks and assistance to affected areas," the prime minister's office said.
At least 48 other countries and the European Union also have offered relief. The nuclear-powered aircraft carrier USS Ronald Reagan arrived off Japan's coast Sunday morning to support Japanese forces in disaster relief operations, the U.S. Department of Defense said in a statement.
Friday's quake is the strongest earthquake in recorded history to hit Japan, according to USGS records that date to 1900. The world's largest recorded quake took place in Chile on May 22, 1960, with a magnitude of 9.5, the agency said.