Defying the odds, rescue workers dug deep into crumbled buildings today, searching for trapped survivors still hanging onto life under the debris four days after Mexico City's devastating earthquake.

Mayor Ramon Aguirre Beltran estimated at 2,000 the number of victims pinned in the rubble of the capital's collapsed structures, most of them given up for dead by now. But firefighters and other rescue workers reported occasional contacts with victims who continued to show signs of life.

A specially trained disaster unit of French firefighters, in the vanguard of an international relief effort now estimated at $2 billion, began working in the rubble using ultrasensitive audio detection equipment.

As aid from abroad flowed in, there were reports from Washington that the International Monetary Fund, which had cut off lending to Mexico on the day of the earthquake, was considering extending up to $400 million in emergency credits.

Aguirre reported that more than 1,600 bodies have been pulled from the city's wreckage in addition to the 2,000 he estimated were still buried in debris. Unofficial death toll estimates were sharply higher. U.S. Ambassador John Gavin said yesterday that the final count could rise to 10,000 or even 20,000.

In Washington, the State Department said the Mexican government had raised the official death toll to 3,461.

Rescue workers interviewed at several collapsed buildings said no organized attempt has been made to gather estimates of the number of persons trapped at each site. This led to speculation that more victims and bodies lie pinned beneath the rubble than the Mexican government has tallied up so far.

The U.S. Embassy said nine Americans have been reported missing since the earthquake. Five others have been confirmed as killed, a spokesman said. They were identified as Mary Elizabeth Vallejo, originally from Cozad, Neb.; her children, Ilse Anne and Alonso Vallejo; Georgiana Merry Yunez, originally from San Antonio, and her 10-month-old son Juan Carlos. Both women were married to Mexicans.

Interior Minister Manuel Bartlett Diaz told Mexican reporters that he and President Miguel de la Madrid have received reports of relatively light damage in the states of Jalisco, Colima, Michoacan, Guerrero and Mexico. Although several dozen persons were reported killed outside the capital, Mexico City suffered by far the highest casualties and damages of the 34 states and one federal district, he said.

Bartlett and de la Madrid met last night with governors from affected states along with rescue officials in Mexico City. The president's office announced that he has canceled a scheduled trip to the United Nations this week to deal with the earthquake crisis.

At the interns' residence of the heavily damaged government medical center complex, the French team used its audio detection equipment to hear the taps of one victim buried under several stories of rubble.

Sgt. Bernard Nouvel, covered with white dust after 24 hours of searching since his arrival from Paris, said that after the first detection, rescuers cried into the rubble asking the survivor to respond by tapping.

"We said to tap three times, and three taps were heard," he said. "Then we asked for four taps, and four taps came up on the device."

Several dozen rescue workers were drilling through the layers of collapsed concrete with pneumatic hammers, poking down in an attempt to create openings toward what they hoped was a survivor who had made the feeble taps in response to their shouts. Nouvel said the slow process of picking through the concrete could go on "for a long time" before an opening was made permitting access.

The French unit also brought dogs specially trained to seek out such survivors and squeeze through small openings to follow moans or scents, he added. Switzerland and the United States dispatched similar canine search teams, officials reported.

Lilian Busio, a 20-year-old university student, stood watching as the drills hammered, as she has stood since an hour after the earthquake struck at 7:18 a.m. Thursday. Her brother, Dr. Francisco Busio, 27, lived in the building while finishing his residency in plastic surgery at the hospital and is presumed to lie, dead or alive, under the concrete, she said.

"I've been here ever since it happened," she added. "But there is nothing."

The international aid flowing into this stricken city included two British Army helicopters and a Hercules transport with lifting and cutting gear from a base in nearby Belize; two Israeli Army teams en route with rubble-clearing and rescue equipment developed for use in Lebanon, and a U.S. Air Force C141 carrying water pumps, power saws, generators and other equipment.

Cuban Foreign Minister Isidoro Malmierca and Health Minister Sergio del Valle arrived in a jet carrying medical supplies. Brazilian President Jose Sarney, en route to New York to address the United Nations, stopped here with medical supplies. Spain and Italy were among other nations sending emergency equipment and supplies.

In Genoa, Italy, Pope John Paul II urged relief organizations to speed emergency aid and prayed for victims of the "appalling pain of the catastrophic earthquake," United Press International reported.

Red Cross and government workers set up supply centers for the thousands of families still camping out in the streets, afraid to return to damaged apartments or left without housing by the tremor. Food and blankets, much of it donated, were distributed at the centers for families with nowhere to go.

City officials announced that schools will remain closed at least through Wednesday. Many families have left the city to stay with relatives in suburbs or other towns. In addition, more than 200 schools were reported damaged.

Benito Arana, a 34-year-old engineer, stood quietly by his furniture and other possessions in the median strip of a large avenue, directly across from the crumbled building that had housed his apartment. On his living room couch lay a cookbook from happier times, entitled "Cakes and Desserts."

"Whom can we pin the blame on?" Arana asked, smiling softly.

Police reported that some looting took place during the night. But these incidents appeared scattered in a city with streets full of shattered store windows and apartment buildings gaping open. Police and soldiers closed off many of the hardest hit areas, allowing passage only to officials or residents.

"The government will act inflexibly with these lawbreakers, since this kind of thing is condemnable, but especially in the conditions we are living in now," Mayor Aguirre said.