The immense scale of the human toll from a devastating Turkish earthquake became more apparent today, as the official number of dead nearly doubled to exceed 7,000 and the government told the United Nations that tens of thousands of people may still lie beneath collapsed slabs of concrete.

A government estimate, based on a preliminary aerial surveillance of the disaster zone, indicated as many as 35,000 people could be buried in the rubble. If the estimate proves accurate, it would catapult the earthquake into the ranks of the most devastating this century, potentially surpassing a 1939 quake that killed 33,000 and has been counted as Turkey's deadliest in recent times.

Turkish officials said the Tuesday morning earthquake will impose huge and lasting costs to Turkish society and the already flagging economy. But fears of additional calamity have not abated. When a series of tremors hit the region today, officials urged millions of residents in Istanbul and the densely populated industrial province of Bursa across the Sea of Marmara to spend the night outdoors as a precaution in case of a major new quake.

As the recovery effort passed into its third day, conditions worsened throughout the earthquake-damaged region in northwestern Turkey. Electricity was still out, water and food were becoming scarce, and local officials were focused mostly on extracting a diminishing number of survivors from the abundant wreckage.

In a new sign of official gloom, Turkey sought United Nations assistance in acquiring at least 10,000 body bags, according to Peter Piazzi, an official with the U.N. Office for the Coordination of Humanitarian Affairs in Geneva.

Temporary morgues were set up at an ice rink in the city of Izmit and at refrigerated military buildings in the vicinity. In the town of Adapazari, officials have buried nearly 1,000 unclaimed bodies after taking their photographs in the hope that family members will eventually turn up. Senior government officials warned of a potential epidemic if additional corpses are not buried immediately.

While the death toll climbed sharply today, other victims were found alive -- including several children and elderly people who emerged from dark concrete caves mostly unscathed after lengthy digging by U.S. and other foreign teams. Still, hopes dwindled that many others will be found alive. In most earthquakes, nearly 85 percent of the survivors are rescued on the first day, and the number typically falls off sharply by the fourth day, in this case Friday.

President Suleyman Demirel said today as he toured Izmit: "I fear the death toll will rise much more."

As relatives and aid workers became increasingly desperate, dogs from a Swiss disaster team that flew to the seaside resort of Yalova were dispatched to sniff inside holes cut in the concrete slabs spilled throughout a 30-foot mound of rubble where a six-story luxury apartment building once stood. The mayor of Yalova, which has a summertime population of 300,000, reported that at least 1,350 bodies had been found so far, and predicted the final toll will exceed 3,000. More than 100 blocks of homes there collapsed.

Yalova was not hit as hard as several cities farther east, such as Sakarya or Golcuk. But damage here was comparable to that experienced in Izmit, which was at the quake's epicenter, the town of Derince, and many other towns and cities along the Izmit Korfezi Bay in the Sea of Marmara southeast of Istanbul. It is this area, in the middle of the most populous region in Turkey, that felt the brunt of the sudden slip of Eurasian and African tectonic plates at the North Anatolian fault line.

Major recovery efforts began only today, as ferries crossing the Sea of Marmara brought more than a dozen construction cranes lashed to flatbed trucks, and a few squads of Turkish soldiers assisted, apparently for the first time, in the search for civilian survivors. An estimated 60 foreign rescue teams arrived with 2,000 members to provide additional assistance, using hydraulic jacks to hoist concrete slabs just high enough to check for signs of life -- or recent death -- with thermal imaging cameras.

Many families continued to flee the region, clogging highways in an attempt to escape the devastation and widening grief.

"The extent of the damage is really incredible," the U.N.'s Piazzi said.

In New York, U.N. deputy spokesman Manoel de Almeida e Silva said the world body has begun studying how to help the government deal with "a prolonged emergency, when we can expect tens of thousands of homeless people in need of assistance."

There was one positive turn of events: Officials reported that a major fire caused by the earthquake at Turkey's largest refinery in Izmit appeared to be burning out. By midday, planes and helicopters had dropped more than 75 tons of fire-retardant foam on the six blazing oil tanks, as 200 firefighters stood by. Fireballs nonetheless continued to billow 300 feet into the air.

"It's a complete disaster," said Ismail Alakoc, the board chairman of Tupras, which runs the plant. But fears that the fire might spread and ignite other chemicals nearby diminished when officials said the tanks would burn themselves out within two days. Alakoc said the fire was under control even though it had not been extinguished.

The International Monetary Fund promised it would give Turkey $325 million to help defray the cost of relief efforts in the seven provinces that experienced damage, which together account for a third of Turkey's economic output. The World Bank also said it would offer $120 million in loans. But early estimates by foreign analysts of the total harm to Turkey's economy were all in the billions of dollars.

Top officials in Yalova said, for example, that the quake had damaged roughly a third of the city's housing, and that at least 1,500 buildings that bear structural cracks caused by the earthquake will have to be torn down and replaced over a lengthy period. In the meantime, they project that as much as 12 percent of the city's year-round population will have to find other accommodations.

Already, virtually the entire remaining populace is sleeping outdoors for fear that their dwellings are not safe. Dozens of streets are blocked by loose electrical wires and debris. A few cars on every other block are smashed almost flat, and many structures have sunk to half the height they had before the quake. Outside the city center, virtually all the apartments or homes in two large resort complexes by the sea have been damaged or destroyed.

The city's fire chief, Naim Ardic, said that with the city's six worst fires largely under control, his top goal is to prevent other authorities from restoring electrical and natural gas service to buildings with damaged utility systems. He said that a comprehensive inspection of every dwelling must be conducted first, to determine which buildings pose an explosion or fire danger -- a task that will take weeks.

Ardic and other local officials have been impressed by the capabilities of the equipment brought in by foreign rescue teams, including the imaging sensors and dogs. He said nothing had been provided by national authorities on the critical first day of the rescue operation.

Correspondent Lee Hockstader in Derince, Turkey, and special correspondent Colum Lynch at the United Nations contributed to this report.