It is the first official representation by Americans since Washington cut ties with Iran after the 1979 revolution.

Teams from 40 countries are helping thousands of residents in the city of Bam who are spending a sixth day outside in harsh conditions.

The quake destroyed 90% of houses in Bam and is believed to have left between 40,000 and 50,000 people dead.

President Mohammed Khatami welcomed the US participation, but stressed that it did not change relations between the two countries.

The 80-strong US team has started setting up a field hospital - the first to operate since Bam's own two hospitals were flattened in the earthquake.

USAid spokesman Dewy Perks told the BBC it was an honour to be helping the people of Iran on behalf of the US Government. It was "not about politics, it's about humanitarian relief", he said.

Iraq has also sent a 55-strong medical team, the head of the Iraq's Red Crescent Society Ezzeddin Chalabi said, and truckloads of emergency supplies have been sent from Kurdish areas of northern Iraq.

Under Saddam Hussein's leadership, Iran and Iraq fought a devastating eight-year war in the 1980s, but relations have improved since his overthrow.

Tons of humanitarian aid have been pouring into Iran, and there have been pledges of $500m in aid from the international community.

Planeloads of tents, blankets, tarpaulins, building materials, medicine, water and food are being stockpiled at Bam's small airport, said UN co-ordinator Ted Pearn.

Most international rescuers in Bam have now abandoned their search for survivors and the focus of the humanitarian effort is being switched towards providing longer-term relief.

However, residents are still asking for teams to search some areas, Mr Pearn said.

Unconfirmed reports on state television said two men and two women had been found alive on Tuesday evening by the Iranian army and transferred to a nearby hospital.

About 2,000 people have been saved from the rubble, while about 30,000 corpses have been recovered.

Bodies are being swiftly buried and the authorities are trying to identify hundreds of unidentified victims, with often-gruesome photographs being shown to relatives on computers at Bam's burial grounds.

The number of dead seems set to rise much higher, but the final figure may never be known as entire families have died and there is no-one left to register them as missing.

"If we consider that, on average, five people lived in each house we can say the death toll will reach 50,000," one Iranian official said.

President Khatami - who has pledged to rebuild Bam's 2,000-year-old citadel "whatever the cost" and to rebuild the city within two years - told reporters he believed the number of dead would be about 40,000.

Iran's health ministry says 14,360 were injured, of whom 8,500 have been admitted to hospital in the nearby city of Kerman, as well as in the capital Tehran and other provinces.