TOKYO — Fighting exhaustion and radiation fears, engineers struggled anew early today to complete the crucial task of hooking a crippled nuclear plant to the electricity grid to help cool down damaged reactors. The official count of dead and missing in the quake and tsunami soared above 18,000, making this Japan's worst disaster since World War II.

TOKYO — Fighting exhaustion and radiation fears, engineers struggled anew early today to complete the crucial task of hooking a crippled nuclear plant to the electricity grid to help cool down damaged reactors. The official count of dead and missing in the quake and tsunami soared above 18,000, making this Japan's worst disaster since World War II.

In the earthquake zone, tears trickled down the cheeks of some survivors and rescue workers who observed a solemn moment of silence at 2:46 p.m. Friday, marking a week since the magnitude-9.0 temblor slammed Japan's northeastern coast.

The quake set off a chain of events culminating in the nuclear accident now ranked at 5 on a 7-point international scale. Still unknown is whether restoring power to the damaged reactors will significantly aid cooling efforts. The full extent of damage to cooling pumps from hydrogen explosions and corrosion from seawater that has been pumped in has not been assessed.

In what many considered an inevitable, and perhaps tardy, move, Japan's nuclear regulatory agency Friday upgraded the severity of the still-unfolding disaster at Fukushima, 150 miles north of Tokyo, from 4 to 5 on the International Nuclear and Radiological Event Scale, meaning it is "an accident with wider consequences." The 1979 Three Mile Island incident, previously considered the second-worst accident in history, was rated a 5 — and it did not cause injuries or a significant release of radiation. The Chernobyl nuclear accident was a 7 — "a major accident" as defined by the scale.

In the earthquake and tsunami zone, hundreds of thousands of people remain displaced. Although the government pledged to accelerate relief efforts, hardship from hunger and cold remained rife. In the quake-shattered city of Miyako, City Hall official Tatsuyuki Kumagai said many of the sheltering survivors were suffering from deep anxiety that frayed at customary Japanese fortitude. "Some cry, others say they're sick of the food. Or they really want to take a bath," he said. The stress, he said, "comes out in different ways."

Establishing a final toll will probably take weeks, but the National Police Agency said the official death count had reached 7,197, exceeding that of the 1995 Kobe earthquake, and the number of those unaccounted for stood at 10,905. Recovery crews have yet to comb through enormous piles of tsunami-deposited debris in some remote areas.

As of early today, about 300 workers were operating inside the 12-mile evacuation zone surrounding the battered nuclear plant, A few dozen were in the complex itself, government and utility officials said. A nuclear safety official said their main objective was to attach power lines to two of the worst-hit reactors.

Other last-ditch measures are under discussion, however, including the drastic option of entombing the complex in cement to stave off a large-scale leak of radiation.

Emergency workers sprayed water toward the reactors for about an hour after midnight, said Kenji Kawasaki, an official from the nuclear safety agency, and more dousing could occur later Saturday. Workers also managed to restart a diesel-powered backup pump that would be used for cooling reactors 5 and 6, the public broadcaster NHK said.

The government and the plant's operator, the Tokyo Electric Power Co., are on the defensive amid rising public anger over what many regard as an incomplete picture of events at the plant, coupled with what has been seen as a feeble relief effort.