Survivors and rescuers in Iran's historic city of Bam (search) were jolted by two frightening aftershocks Monday morning that caused some of the few remaining walls to tumble and left a rising cloud of dust over the remains of the city's ancient fortress.

More than 21,000 bodies have been retrieved since Friday's 6.6-magnitude earthquake (search) shook the city and surrounding region in southeast Iran, according to provincial government spokesman Asadollah Iranmanesh.

"Many, many more people remain buried under the rubble, increasing fears of a much greater death toll at the end," Iranmanesh said.

Some officials have expressed fears the death toll could rise as high as 40,000. Iranmanesh said Sunday that 10,000 people were hospitalized.

Iran's Supreme Leader Ayatollah Ali Khamenei (search) made an unannounced visit to Bam Monday morning to inspect relief efforts and President Mohammad Khatami (search) was expected to arrive later in the day.

Hopes of finding more survivors faded as sunrise Monday marked 72 hours since the quake hit, entombing thousands of sleeping residents in their homes. Experts say 72 hours is generally the longest people can survive if they are trapped in rubble.

Rescue workers from around the world joined Iranians in searching through powdery debris that left little room for air pockets, which could allow people to survive while awaiting help.

James Brown, spokesman for a British rescue team, said a human being can survive three to four days without water, and three to four weeks without food.

"There is always hope for survivors," Brown said Monday.

Only one man was pulled alive from the rubble Sunday, Iranmanesh said. A day earlier, officials reported freeing 150 survivors.

Interior Minister Abdolvahed Mousavi Lari said Sunday that the search for survivors would probably end Monday night.

Ted Pearn, coordinator of U.N. relief operations in Bam, said 1,400 international relief workers were in Bam, part of 35 teams from 26 countries.

Planes from dozens of countries, including the United States, have landed in the provincial capital of Kerman with relief supplies, volunteers and trained dogs.

U.S. military C-130 cargo planes were among the arrivals, despite long-severed diplomatic relations and President Bush's characterization of Iran as being part of an "axis of evil" with Iraq and North Korea.

Interior Minister Lari said Iran accepted U.S. government help and not Israeli help because Tehran considers the United States a legitimate government, but opposes Israel for its actions against Palestinians in the West Bank and Gaza Strip. Israel "is a force of occupation," he said.

As for Americans, Lari said. "I believe it is possible that they have a humanitarian sensibility in such a dramatic situation."

The traditional sun-dried, mud-brick construction of the houses doomed many occupants, as it has for centuries in quake-prone Iran. Heavy roofs, often sealed with cement or plaster to keep out rain, sit atop mud-brick walls that have no support beams. When walls crumble, roofs smash down, leaving few air pockets and crushing or suffocating anyone inside.

Mostafa Biderani and his wife, Zahra Nazari, wept in front of a destroyed police station in the center of Bam, slapping their faces and beating their chests in an Islamic expression of grief.

"I pulled my son out of the rubble this morning," said Biderani, who drove from Isfahan, 470 miles to the northwest.

"In these conditions, we are not optimistic of finding anyone alive. Hopes are dwindling fast," said Barry Sessions of Britain's Rapid-UK rescue group, which did not find any survivors in 24 hours of searching.

"The earthquake reduced most of the buildings to something like talcum powder. Many of the casualties suffocated and there are few voids or gaps left in the buildings where we would normally find survivors."

His thoughts were echoed by other relief workers.

Luca Spoletini, spokesman for the Italian Civil Protection, said its teams found nothing but corpses after a day spent probing the rubble.

Describing a visit to Barazat, a town with a population of 20,000 a few miles outside Bam, Spoletini said, "There is nothing any more. Not one single house, not one single building stands upright. It is like the Apocalypse. I have never seen anything like that."

By Saturday night, enough tents had arrived to accommodate the thousands of homeless. Looters were also out, grabbing food from warehouses and grocery shops. Police tried to control them by shooting in the air.

Bam was best known for its medieval citadel, considered the world's largest surviving mud fortress. Most of the 2,000-year-old fortress, including a massive square tower, crumbled like a sand castle when the quake hit.