At least nine firefighters and rescue workers were killed in Buenos Aires when the wall of a burning warehouse collapsed on top of them, officials said.

The wall fell as firefighters were using an electric saw to cut through a huge iron gate at the warehouse, which housed archival bank records.

Authorities said the dead included six members from a federal police firefighting unit, a volunteer firefighter, and two members of Argentina's civil guard.

Argentina has declared two days of national mourning after a rescue crew was killed in a warehouse fire. ( AFP: Gustavo Fidanza )

Speaking outside the warehouse, where the fire was still not under control after seven hours, security secretary Sergio Berni said eight people had also been injured, including five who were still hospitalised with multiple traumas.

However, "so far there are no reports of missing persons," Mr Berni added, and a company spokesman said warehouse employees were all safe.

Mr Berni had earlier said said several other first responders who were badly injured in the accident were "fighting for their lives."

Emergency workers who arrived to aid those crushed under the debris "found a lot of bodies covered by rubble, some with multiple serious injuries," said Alberto Crescenti, head of the public medical service.

"Unfortunately, most of the victims died at the scene," he said.

One grieving civil guard officer at the scene said he was devastated by the loss of his comrades.

"What happened to us is a terrible tragedy that never should have occurred," he said.

"We are sad - and angry. We work for next to nothing, but nevertheless put on a brave face," he said, adding that under such conditions "we just can't continue."

An aerial view of the burning warehouse ( AFP: Nicolas Garcia )

Officials said they do not know what caused the blaze in the 19th century building in southern Buenos Aires.

Iron Mountain, the US company that owned the warehouse, said in a statement it was "deeply saddened by the deaths of the brave rescuers who ... arrived quickly to save our facilities."

"All Iron Mountain employees are safe," the statement said, adding that the building had been equipped with smoke detectors and a fire extinguisher system.

President Cristina Kirchner's government and Buenos Aires mayor Mauricio Macri declared two days of national mourning over the tragedy.

AFP