ANKARA: At least 20 firefighters were killed when a 17-storey Tehran commercial building collapsed on top of them as they tried to put out a blaze, Iranian state television quoted the city's mayor as saying on Thursday.

Soldiers, sniffer dogs and rescue workers were searching the ruins of the Plasco building after it crashed down in a giant cloud of dust.

The collapse was shown live on state television and one witness described it as "like a horror movie".

"At least 20 firefighters who were trapped under rubble have died," Mayor Mohammad Baqer Qalibaf said.

"They are martyrs. They lost their lives when trying to help people."But Tehran Fire Department spokesman Jalal Maleki told the broadcaster: "I cannot confirm the death of around 20 firefighters ... but some of them have been killed. The rescue operation continues."Tehran’s Governor Hossein Hashemi told state TV rescue workers were opening three separate routes to access those trapped.

"One of those trapped in the building sent a text message saying five of them are alive," Judiciary website Mizan reported. State TV said at least 78 people, including 45 firefighters, were injured.

Most were taken to hospital and many quickly discharged, it said.The semi-official Tasnim news agency said troops had been sent to help dig through the ruins. It said one of the first firefighters to be reached had asked to be let back inside to save his colleagues.

The agency quoted an official in the Tehran governor's office as saying an electrical short-circuit had caused the fire, but there was no immediate confirmation of this.President Hassan Rouhani ordered an investigation and compensation for those affected.