Iranian officials now say at least 20,000 people were killed in Friday's earthquake that hit the southeastern city of Bam.
Officials say 50,000 others were wounded. The historic city of Bam has a population of 80,000 people.
Rescue workers have moved thousands of injured Iranians to hospitals in nearby towns. But local residents are complaining that the government is not providing enough tents, water and food.
Iran has appealed for international aid to deal with the disaster. Hours after the quake, Russia sent two planes to Iran carrying medical and rescue personnel as well as search dogs.
President Bush offered condolences to Iran and said Washington is ready to help. But the White House did not offer any details about what aid the United States has offered.
The quake hit Bam at about 5:30 in the morning local time (two hours UTC) when most residents were sleeping.
Iran said the quake measured 6.3 on the Richter scale, but the U.S. Geological Survey placed the magnitude higher, at 6.7.
Earthquakes occur frequently in Iran, which is crossed by several major fault lines. A 1990 earthquake in northwestern Iran killed 35,000 people.
Observers say most of the buildings in Bam were not earthquake-proof, being built out of simple materials like mud bricks and clay. Information for this report is provided by AFP and Reuters.