Tehran prosecutor-general Saeed Mortazavi said around 20 construction workers were in the building at the time, suggesting the death toll could rise.

He said the owners had been told to tear the building down after it started to collapse about a month ago and that the workers had been trying to salvage window frames and equipment.

"When the building collapsed early this morning the entire workforce of around 20 were in it and so far no one has been rescued," Mortazavi told state radio.

A Reuters witness said the building had collapsed into a heap of rubble. Rescue workers were trying to shift the debris while sniffer dogs searched for victims and survivors.

"We have so far unearthed three bodies," said Behrouz Tashakkor, spokesman for Tehran's fire department. "How many more there are under the rubble, I don't know. We are continuing to remove the rubble."

Tehran, a sprawling city of some 12 million people, has seen a boom in construction in recent years, with builders rushing to keep up with demand in the world's fourth-largest oil producer.