Japanese rescue workers carry the body of a tsunami victim in devastated town of Otsuchi March 14, 2011. In the town of Otsuchi in Iwate prefecture, 12,000 out of a population of 15,000 have disappeared. (DAMIR SAGOLJ/REUTERS)

Overwhelmed by a still-growing catastrophe, Japanese authorities struggled Monday to reach buried survivors and the missing, faced roadblocks in delivering aid and raced to contain an expanding nuclear emergency.

Prime Minister Naoto Kan called the crisis the country’s toughest challenge since World War II and said that decimated towns along the northeastern coastline were not yet getting the food and supplies they needed.

A series of unstable nuclear plants across the country threatened to compound the nation’s difficulties, which started with Friday’s double-barreled disasters: first an 8.9-magnitude earthquake, then a tsunami. At the Fukushima Daiichi nuclear plant, one containment building housing an overheated reactor had already exploded. A second explosion, about noon local time Monday, destroyed an outer building at another of the plant’s reactors.

Officials said a third reactor at the six-reactor facility had lost its cooling capacity, and the U.S. Seventh Fleet, stationed 100 miles offshore, repositioned its ships and aircraft after some if its personnel came into contact with radioactive contamination.

With a government spokesman saying that the reactor units could be in partial meltdown, an alarmed public struggled to understand the safety implications of trace radiation leakage, even as the government said that public safety was not in danger.

The economic toll of the disaster was evident Monday as Japanese stock markets fell more than 6 percent. After the first full day of trafing since the earthquake and tsunami, the Nikkei index of major Tokyo Stock Exchange companies dropped 633.94 points to close at 9,620.49 — wiping out this year’s gains and hitting its lowest level in four months. Japan’s central bank said it will put $183.8 billion into money markets to stabilize the financial system.

People here are frightened by what they can’t see and shocked by what they are seeing. Entire towns have been swamped, and the hardest-hit areas still don’t have what they need, according to those in shelters and those organizing relief efforts. There’s not enough food, not enough water and, in many places, no heat. Tens of thousands remain missing, beyond the reach of rescue workers.

As the official death toll surpassed 1,000, the police chief of Miyagi Prefecture, among the hardest-hit regions, said Sunday that there is “no question” that at least 10,000 people in the prefecture of 2.3 million are dead. Other prefectures in the northern part of Japan’s main Honshu island could face similar tolls.

One Red Cross official said that in the Pacific coast town of Ishinomaki, the local hospital feared it was about to run out of food and milk for babies. Most gas stations along the main roads heading north from Tokyo do still have gas — but lines snake around for several blocks.

Amid all this, Japan has mobilized 100,000 troops — a doubling of the force that had been called into action a day earlier — who are racing to rescue those in towns that were swept away. Kan said the government was exploring the possibility of delivering food by sea or air, given the problems with roads en route to the north.

Because the damaged nuclear plants have left the country with a depleted energy supply, Japan on Monday began rolling blackouts across a slice of the country that includes Tokyo, home to about 13 million. Kan approved the measure even though the scheduled outages — a first for modern Japan — will impede business as this economically wounded country returns to work.If the country cannot save energy, officials said, unplanned blackouts could turn much of the country dark and add one more barrier for rescue workers.

Updated Google Earth imagesshowed aerial-view photos of towns where a mosaic of colorful rooftops had been churned into mush. In the coastal town of Rikuzentakata, in hard-hit Iwate Prefecture, only 5,900 of the town’s 23,000 residents have taken shelter, according to the Kyodo news service. The rest were unaccounted for.

Youka Ishi, who works at a town office two miles from the Miyagi Prefecture coast, said that roughly 2,700 buildings closer to the water “have been swallowed by the wave, and there is nothing left.”

“I know, through my work as a welfare worker, about 40 or 50 elderly people in that area,” Ishi said. “Not one of them have I been able to contact, or even see just the face of.”

In Sendai city, which was hit hard along its coast, as many as 500 people stayed Sunday night in the undamaged prefectural office.Those who went outside could still see a fire smoldering somewhere along the coastline.

Cameron Peek, 23, an American who teaches English in Sendai city, was staying at the shelter. “We have enough space and food,” he said. “People have been taking cardboard boxes from convenience stores and making beds. Everybody seems pretty spent.”

Japan’s government on Sunday broadened efforts to cope with the disaster, with its central bank approving more than $650 million in loans to banks in the hardest-hit areas. Japan’s parliament was temporarily suspended as officials deal with the crisis.

Authorities here have received criticism for their handling of the nuclear emergency — particularly their communication to the public. In a Sunday editorial, the Yomiuri Shimbun, Japan’s largest newspaper, took aim at the government’s response time in explaining, or even acknowledging, the Saturday explosion at the Fukushima Daiichi plant. Only five hours after the explosion did Chief Cabinet Secretary Yukio Edano say that no radioactive substances had been leaked as a result of the blast.

Engineers are now rushing to stabilize at least two overheated reactors by injecting seawater into their cores. Japanese officials say minute quantities of radiation have already been released, and hundreds of people in Fukushima Prefecture are being tested for radiation exposure. Nuclear engineering experts say the evidence points to severe damage to the uranium fuel rods in Fukushima Daiichi unit 1, a situation they classify as a partial meltdown. If the cores cannot be cooled sufficiently, a total meltdown could ensue, exposing the fuel rods to the outside air and further spreading radiation.

The government also said it would distribute potassium iodide pills to those near the plant.

The United States has sent a disaster team to Tokyo that includes both government nuclear experts and rescue squads. U.S. troops and a nuclear-powered aircraft carrie have already been dispatched to assist.

Meanwhile, the State Department said that U.S. citizens, including government workers on nonessential business, “should avoid travel to Japan at this time.” In the three days since the offshore earthquake, Honshu island has suffered hundreds of aftershocks, with experts expecting more in the coming weeks.

Along the arteries that connect Tokyo with towns that are most in need of supplies and assistance, travelers encountered mudslides and fuel shortages, even less than 100 miles north of Tokyo.

The shelves of the Family Mart convenience store in Kagamiishi, Fukushima Prefecture, were wiped clean, save for alcohol and condiments.

At the store, Miki Arai packed what few supplies were available into a cardboard box. Arai had scheduled a vacation from his job as an IT engineer in Tokyo when the earthquake hit two days earlier. He left the city Sunday morning hoping to help in Sendai however he could — a one-man rescue team. He took trains as far north as he could, then rented a motorbike. As he latched the box of supplies to the back of his bike, Arai explained that he had given up on Sendai.

“It’s too far, there’s no gas to get back, and it’s easier to reach to evacuated tsunami victims closer to Fukushima,” Arai said.

Staff writers Rick Maese in Fukushima and Howard Schneider in Washington contributed to this report.