Japanese officials ordered people near the Fukushima No. 1 plant -- around which a 12-mile evacuation zone had already been carved out -- to stay indoors after a hydrogen blast in the containment building of one of its six reactors, similar to one that occurred Saturday in a separate reactor.
Cabinet secretary Yukio Edano, speaking in a live TV broadcast, said it was believed that the reactor remained intact and "we think that the possibility of a massive radiation emission is low."
Photos: Scenes of earthquake destruction
But the apparently intensifying nuclear crisis sent a wave of fear and anxiety through the quake-battered country, as powerful aftershocks from Friday's temblor, the worst in Japan's recorded history, continued to rattle cities and towns. Earlier, the government reported that radiation levels had again risen above legal limits outside the Fukushima complex, about 150 miles north of Tokyo, where authorities have been pumping seawater into two overheated reactors to try to cool them down.
The use of seawater was considered a drastic emergency measure — a "Hail Mary" improvisation, by the characterization of some experts — and it emerged that early Monday, even that had briefly failed. Japan's nuclear safety agency said at about 1 a.m., the seawater injection halted for about 2 1/2 hours because the tanks being used went dry, and that stoppage triggered rising pressure in the reactor container prior to the explosion.
A Tokyo Electric Power Co. official said six people were missing after the blast at the Fukushima nuclear plant. Several other nuclear installations were under close watch for potential problems.
The International Atomic Energy Agency said its lowest classification of emergency had been declared at a separate plant outside the northern town of Sendai. But Japan's nuclear safety agency said that although there had been a rise in radiation the reactor's cooling system was functioning adequately.
Across a wide swath of earthquake-hit territory, hundreds of thousands of shivering survivors roused themselves from a third cold night spent huddled in darkened emergency centers, cut off from rescuers, aid and electricity. At least 1.4 million households had gone without water since the quake struck and some 1.9 million households were without power. Rolling blackouts to conserve energy were scheduled across much of the country.
In Tokyo and other large cities outside the quake zone, the first full workday since Friday's temblor began with delays and disruptions. Many of the train lines that normally run between Tokyo and outlying suburbs and surrounding cities were either running far less frequently than normal or not running at all. Subway and train lines crisscrossing the capital were also curtailed.
With fears about how the world's third-largest economy would weather the ongoing fallout from the massive quake, Japan's main stock index, the Nikkei, plunged by 6% in early trading. The central bank said it would inject a record $182 billion into money markets to try to stabilize the financial system.
A full reckoning of deaths and damage could take weeks, but the picture grew grimmer with each passing hour. Japan's Kyodo News agency reported the discovery of an additional 2,000 bodies at two separate locations in Miyagi prefecture. The official death toll — expected to be dwarfed by a final full count — stood at 1,598. The official rolls listed 1,720 people missing, but many thousands more are unaccounted for.
In Miyagi, with a population of 2.3 million, at least 10,000 residents were killed, police spokesman Go Sugawara told the Associated Press late Sunday.
Videos show nuclear plant explosion
An international rescue effort gathered force, with teams arriving from China, New Zealand, Germany and the United States, among other nations. Eighty-eight governments and six international institutions have offered assistance with recovery efforts, the Japanese Foreign Ministry announced. Even Bangladesh offered to help — an unaccustomed reversal for Japan, one of the world's biggest donors of foreign aid.
Among those arriving were a Los Angeles County Fire Department search-and-rescue team at Misawa Air Base about 400 miles north of Tokyo with 74 tons of equipment, including swift-water rescue gear and six search dogs, spokesman Don Kunitomi said.
With 100,000 Japanese troops taking part in the rescue effort, the Defense Ministry said it was considering the formation of a volunteer rescue brigade, consisting of thousands of additional helpers who could be dispatched within days.
There were some dramatic rescues of tsunami survivors Sunday, including that of a 60-year-old man who had been waiting for help since he was swept out to sea Friday. Hiromitsu Shinkawa was spotted by rescuers at 12:40 p.m. ,nine miles off shore by the crew of a Japanese Maritime Self-Defense Force destroyer, according to Jiji Press.
Shinkawa, from the devastated city of Minamisoma, was conscious and in "good condition" after the rescue, Japanese officials said.