International aid is pouring into Iran to help victims of a powerful earthquake that flattened the ancient city of Bam on Friday, killing an estimated 20,000 people and injuring 30,000 others.

The first American aid shipment to Iran arrived early Sunday aboard two U.S. military transport planes that landed in the city of Kerman, about 200 kilometers from Bam.

Four other such flights are planned, and the United States is also sending medical and disaster-coordination teams and search-and-rescue experts to the earthquake zone.

Associated Press reports a second American flight also arrived in Kerman province in southeastern Iran. Iran's official IRNA news agency says 25 countries are taking part in an aid airlift, and 45 foreign planes have arrived already.

Iran's interior minister, Abdolvahed Mousavi Lari, says 15,000 bodies have already been buried, and he expects the final death toll to pass 20,000. Reporters at the scene in Bam say many bodies are being buried in mass graves.

The centuries-old mud-brick citadel in Bam was one of Iran's best-known tourist attractions, but there are only incomplete reports about how many foreigners were caught by the earthquake. U.S. officials say one American tourist was killed and another was injured.

Local and international rescue teams are digging through the rubble of homes collapsed by the earthquake, which struck before dawn on Friday, while many people were asleep. No more than 200 people have been reported to have been found alive since Saturday, and there are growing concerns that time is running out for anyone who may still be alive beneath the wreckage.

Rescue efforts are being hampered by freezing overnight temperatures, as well as a lack of power, water and communications. Relief workers have set up tents, but thousands of homeless people still are sleeping outdoors. Reporters in Bam for Reuters say there has been some looting of relief supplies by gangs of young men armed with pistols.

Traffic gridlock is also a problem. Many people are trying to flee the region as aftershocks spread fear, while others have been trying to reach Bam to help in rescue efforts or search for family and friends.