DAR ES SALAAM, Tanzania -- A building under construction collapsed in the center of Tanzania's commercial capital Dar es Salaam on Friday and rescuers searched for survivors under the rubble, with conflicting reports about the number of dead.

A senior police officer initially told reporters 15 people were killed and two people were pulled out alive. Hours later, the mayor for central Dar es Salaam, Jerry Silaa, said two people were killed and 17 survivors had been found.

Emmanuel Herman / Reuters

An aerial view shows bystanders watching rescuers search for survivors in the rubble of a collapsed building in Tanzania's commercial capital on Friday.

The building, in the Kariakoo district, was at least 12 stories high. Witnesses said some cars were crushed in the collapse and people were using their hands to pull away masonry.

Rescue workers said they heard the voices of people trapped, possibly including boys who had been playing soccer nearby when the building collapsed. Some witnesses said construction workers may also have been trapped.

Tanzania's buoyant economy has fuelled a construction boom, and Kariakoo in particular has been a focus for building. The speed of construction has raised concerns over standards.

Senior government officials also gathered at the scene.

Emmanuel Herman / Reuters

Rescuers search for survivors in the rubble of a collapsed building in the Kariakoo district of central Dar es Salaam, Tanzania on Friday.

"This is a big catch, the biggest ever single seizure of ivory at the port of Mombasa," said Kiberenge Seroney, the port's police officer in charge of criminal investigations. "We fail to understand where one gathers the courage to park such enormous quantities of ivory, hoping that they can slip through our security systems."

Poaching is a growing problem for sub-Saharan African countries reliant on rich wildlife in their game reserves to draw foreign tourists.

Heavily-armed criminals kill elephants and rhinos for their tusks, which are used for ornaments and in some folk medicines. Most of the elephant tusks smuggled from Africa ends up in Asian countries, according to police.

On Jan. 5, poachers killed a family of 11 elephants in the biggest single mass shooting of the animals on record in Kenya, wildlife officials said.

Gitau Gitau, an assistant commissioner with the Kenya Revenue Authority, said paperwork accompanying a container at the port of Mombasa declared it contained decorative stones.

"But when we opened it we found elephant tusks," said Gitau as he displayed the ivory. "The ivory was originating from Rwanda and Tanzania and was to be exported to Indonesia."

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