Sendai, Japan (CNN) -- Rescuers began a fourth day picking through the rubble of earthquake- and tsunami-ravaged northern Japan Monday, searching for survivors, as the country's prime minister called on people to pull together and face sacrifices following Friday's historic disaster.
"In the 65 years after the end of World War II, this is the toughest and the most difficult crisis for Japan," Prime Minister Naoto Kan told reporters at a televised news conference Sunday.
"We Japanese had a lot of difficulties in the past, but we were able to overcome those difficulties to reach this peaceful and prosperous society we have been able to build," Kan said. "So with regard to the earthquake and tsunami, I am confident that the Japanese people can be united to work together."
He added, "I ask each one of you, please have such determination, and deepen your bond with your family members, your neighbors and the people in your community to overcome this crisis so that Japan can be a better place. We can do it together."
The cooperation Kan called for will include accepting rolling blackouts in some areas to preserve electricity as emergency workers try to repair power plants damaged by the quake. 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 the capital's Rainbow Bridge.
The death toll from the tragedy neared 1,600 on Sunday, with more than 1,900 injured and nearly 1,500 missing, the National Police Agency said. The 8.9-magnitude earthquake tore through Japan on Friday, triggering massive waves that ravaged everything in their path.
So far, about 15,000 people have been rescued, the Kyodo News Agency reported Monday, citing Kan.
Japanese officials raised the quake's magnitude to 9.0 on Sunday, but the U.S. Geological Survey kept its magnitude at 8.9.
Residents Sunday hoped they are spared a potential further catastrophe: a widespread release of radiation from nuclear power plants damaged by the quake. Workers were flooding two of the six reactors at the Fukushima Daiichi plant in a last-ditch attempt to head off a catastrophic meltdown of the reactor core.
"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 nothing for a radiation leak because no one teaches you what to do in that case," she said.
The quake and tsunami disabled the coolant systems at Fukushima Daiichi, which is about 260 km (160 miles) north of Tokyo. Japanese authorities have said there is a "possibility" that a meltdown has occurred in the damaged reactors, but said that there were no indications of dangerously high radiation levels in the atmosphere.
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.
At least 11 workers have been sickened or injured at the Daiichi plant in the quake's aftermath, the plant's parent company, Tokyo Electric Power Company, said Sunday. Two employees are missing, the company said.
Japan also declared an emergency at a second nuclear plant, in Onagawa, where excessive radiation levels have been recorded since Friday, according to the International Atomic Energy Agency. However, radiation levels had dropped to normal levels by early Monday, and the "current assumption" of Japanese authorities is that the increased radiation levels may have been caused by material released from Fukushima Daiichi, the U.N. nuclear watchdog agency reported.
Meanwhile, in neighborhoods swallowed by the walls of water generated by the quake, rescuers and shell-shocked residents scrambled to reach survivors Sunday. Japanese troops went door-to-door in the city of Ishinomaki, hoping to find survivors -- but finding mostly the bodies of elderly residents.
In one coastal town alone -- Minami Sanriku, in Myagi Prefecture -- 9,500 people, about half the town's population, were unaccounted for.
And 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) west of the earthquake's epicenter. Scores lined up at the few gas stations, drug stores and supermarkets that were open, and 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."
The worst 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 were 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.
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 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.
CNN's Tom Watkins, Anna Coren, Kyung Lah, Paula Hancocks, Brian Walker, Kevin Voigt and Sean Morris contributed to this report.