At least 5,000 people are believed dead after an earthquake struck the historic Iranian city of Bam on Friday but officials estimate the final death toll could reach 20,000.
The number of injured was put at 50,000, most of whom were homeless during a night of freezing temperatures.
The quake flattened well over half and perhaps as many as three-quarters of the buildings in the picturesque city of Bam - known for its giant medieval fortress complete with towers and domes made of mud-brick.
"The disaster is far too huge for us to meet all of our needs," President Mohammad Khatami said.
Speaking on television, Mr Khatami called for international assistance. Russia, Europe and the United Nations immediately pledged to help rescue efforts with food, blankets and volunteers. Ireland has pledged 1 million.
IRNA, the state news agency measured the quake at 6.3 on the Richter scale and the U.S. Geological Society put the figure at 6.5.
It ripped through Bam around 5.30 a.m. local time, when most of the city's residents were sleeping.
The city's citadel, one of the largest mud-brick buildings in the world, was badly damaged.
"The entire town will need to be rebuilt," Governor Ali Shafii told state television. "You look everywhere, you will see piles of rubble and ... many of the bodies are still under the rubble. People are suffering from cold, and there is an immediate need for tents, medicines, food."
Pictures on Iranian television showed tearful children digging for their parents amid men rooting in the debris with their bare hands.
Two of Bam's hospitals reportedly crumbled, killing most of the staff. The rest of the hospitals overflowed with patients huddled under blankets.