Six steel girders weighing about 20 tons each disappeared mysteriously from a warehouse belonging to the Rio de Janeiro city government, and the mayor - Eduardo Paes - on Wednesday called the apparent theft an "unbelievable" case.

The girders - which are each 40 meters (131 feet) long - were removed from a downtown Rio viaduct that was demolished in accord with the revitalization plan for the city's port area and had been stored for the past two months at a warehouse in the nearby neighborhood of Caju, from where they disappeared.

Paes said that those responsible for the demolition work just noticed the theft on Wednesday and nobody can come up with a hypothesis regarding how the girders were removed without any of the guards seeing it, given that transporting them is a complicated logistical operation.

The mayor said that the Porto Novo consortium, which received the concession to perform the port revitalization work, will have to reimburse the city government if the whereabouts of the enormous pieces of steel cannot be determined.

"The (consortium) is the one that is performing the work and (is) the party who had to remove the girders and watch over them before delivering them to the city government. It's absurd. It's not possible to imagine that some girders of that size and weight could disappear overnight," he said.

"I can't imagine what could have happened. I was surprised at that news," the mayor added.

The girders were part of a viaduct that ran around the entire Rio port and that was built during the 1970s with gigantic pieces of steel.

Despite the fact that they have been exposed to the effects of the seawater for 35 years, the girders showed no signs of corrosion or rusting.

Truckers working in the Caju area said that the thieves would have needed almost a week to cut the girders into pieces before removing them because carting away an entire girder would have required the use of two cranes and special trucks.

Witnesses said that they had recently seen people working calmly around the girders but they did not call the police because they thought that they were city government personnel. EFE