An official with the Imperial Household Agency said that Akihito had never before delivered a nationally televised address of any kind, not even in the aftermath of the Kobe earthquake in 1995 that killed more than 6,000 people. The address was videotaped.
The remarks on Wednesday afternoon were the first public comments from Akihito, 77, since a devastating earthquake and tsunami struck northern Japan last Friday, and they underscored the urgency of multiple crises confronting the country. Akihito expressed his concern for the survivors of the disaster and thanked the rescue teams working under difficult conditions in the north.
A huge relief operation continued as hundreds of thousands of people prepared to spend a sixth night in temporary shelters amid freezing temperatures.
Before the emperor's address, the crisis took another turn for the worse. Authorities said a containment vessel in a second reactor unit at the stricken Fukushima Daiichi plant in northeastern Japan might have ruptured and appeared to be releasing radioactive steam. That would be the second vessel to be compromised in two days.
A spike in radiation levels at the plant suspended some critical efforts to pump water into several reactors to keep them cool. Earlier in the morning, the company that runs the plant reported that a fire was burning at a different reactor.
Japan's neighbors watched the nuclear crisis anxiously. Officials from the Hong Kong Observatory -- about 1,800 miles south of Tokyo -- said they were monitoring radiation levels and wind directions from Japan. Sampling of air for radiation over the southern Chinese territory had been increased and levels remained normal, said C.C. Chan, a senior science officer at the observatory.
Various nations across East Asia have said they would step up inspections of food imported from Japan.
The death toll climbed inexorably. More than 3,600 people were confirmed dead and more than 7,800 remained unaccounted for by Wednesday afternoon. Authorities say the number of dead is likely to exceed 10,000.
Aftershocks kept people across northern Japan on edge Wednesday. The United States Geological Survey recorded 54 earthquakes by midafternoon, four of them with magnitudes higher than 6.0. A strong morning shock caused buildings to sway in central Tokyo for about 30 seconds.
An estimated 440,000 people are living in makeshift shelters or evacuation centers, officials said. Bitterly cold and windy weather compounded the misery as survivors endured shortages of food, fuel and water.
Weather forecasters predicted a cold front moving into the region would send the overnight temperatures in northeast Japan below freezing, and the government said the cold posed a health risk for evacuees.
Rescue teams from 13 nations continued to search for survivors, and more nations were preparing to send teams. Helicopters shuttled back and forth, part of a mobilization of some 100,000 troops, the largest in Japan since World War II, to assist in the rescue and relief work. A no-flight zone was imposed around the stricken nuclear plants.
Some foreign embassies have suggested that their citizens head south, away from Fukushima Prefecture -- which is near the epicenter and home to the worst of the crippled reactors -- or leave the country, directives that have led to a rush of departures this week at Narita Airport, Tokyo's main international gateway. On Wednesday, the French Embassy in Tokyo urged its nationals to move south or leave the country. In a message on its Web site, the embassy also said Air France had been asked to provide planes to ferry French citizens out of Japan.
A number of foreign airlines have suspended flights to Tokyo and have shifted operations to cities farther south, and some expatriates left on Tuesday.
The United States ambassador to Japan, John V. Roos, issued a statement on Wednesday addressing the growing public unease over the nuclear crisis. American officials agreed with the Japanese government's advice urging people living within about 12 miles of the Daiichi plant to evacuate, and within about 18 miles to remain indoors.
"Let me also address reports of very low levels of radiation outside the evacuation area detected by U.S. and Japanese sensitive instrumentation," Mr. Roos said in the statement. "This bears very careful monitoring, which we are doing. If we assess that the radiation poses a threat to public health, we will share that information and provide relevant guidance immediately."
The Japanese stock market regained some poise despite the continued uncertainty, clawing back some of the massive sell-off of the previous two days. The benchmark Nikkei 225 index closed up 5.7 percent, and the broader Topix gained 6.6 percent.