The death toll from Friday's 6.6-magnitude quake that shook the ancient city of Bam rose to 28,000, according to Ted Peran, coordinator of UN relief operations. At least 10,000 people were believed injured.

"We have gone out of the rescue phase and entered the humanitarian relief phase of the operation," Peran said. "There's always hope of pulling more survivors out ... but the window of opportunity is closing rapidly."

Some international rescuers headed home, saying they were frustrated at their inability to save lives. There were fears the number of dead could rise as high as 40,000 after Bam on Monday passed the critical mark of 72 hours after the quake, the longest period people are expected to survive in rubble.

Occasionally, people survive longer if they are trapped in a pocket with air to breathe, though Bam's traditional architecture sharply limited that possibility. The city's mud-brick houses, constructed without supporting metal or wooden beams, crumbled into small chunks and powder-like dust.

"We did not find anyone alive," said Steve Owens of the charity British International Search. His team spent 14 hours traveling less than 125 miles on a jammed road to Bam and reached the devastated city too late to help.

"We were a day late getting to the site," Owens said, waiting Tuesday at the airport in the provincial capital of Kerman for a flight back to England. "When things like this happen, there should be ways to get teams in quicker. It's frustrating."

Russia's Emergency Situations' Ministry said that its 150 rescuers would return to Moscow on Tuesday and that a plane carrying humanitarian aid would be sent to Iran on Wednesday.

At the peak of rescue efforts, 1,700 international relief workers from 30 countries had converged in Bam, Peran said. By Tuesday, the number of rescuers dropped to about 1,500 after seven teams returned home.