TOKYO - 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.