Mining giant BHP Billiton on Thursday vowed there would be a transparent investigation into a dam rupture at a mine in Brazil as the death toll rose to 11.

Two dams at a waste collection pond at the Samarco iron ore mine broke on November 5, spewing a deadly wall of mud and water that is being tested for toxic elements. Most of the village of Bento Rodrigues was flattened, with hundreds of people affected.

The mine is owned by BHP, the world's number one mining company, and Brazil's Vale, with the Australian-based behemoth admitting it was facing one of the toughest times in its 130-year history.

"Tragically, at this stage we believe 11 people are dead, and eight remain missing," chairman Jac Nasser told the company's annual general meeting in Perth, where protesters held banners reading “BHP: Mud and Profit”. The previous death toll was 10.

"We are deeply sorry for all of those who have been impacted by this tragedy, in particular for the friends and families of those who died and those who are missing, as well as those who have lost their homes and feel uncertain about the future," added Nasser.
Samarco pledged Monday to pay an initial US$260 million in damages related to the deadly torrent and chief executive Andrew Mackenzie vowed BHP "will fully play our part in helping Samarco reconstruct homes, community and spirit".

Although the mine is a big employer and Vale is one of Brazil's biggest companies, anger at the sudden catastrophe has been intense, with experts predicting it will take the ecosystem decades to recover.

Deutsche Bank has estimated the overall clean-up cost could top US$1 billion.

Nasser said the findings of an external inquiry into how the disaster happened would be made fully public so resource companies worldwide "learn from these tragic events".

"I commit to you that we will find out what went wrong," he said.

"Together with Vale we have agreed to commission an external investigation into the collapse of the tailings dam. It will be some time before this investigation concludes, but when it does, you have my commitment that we will publicly release the findings.

"We will also share the results with other resource companies."

In the meantime, Samarco operations will remain suspended with work under way to reinforce the dam structures and stabilise the area.

Brazilian authorities declared a state of emergency Tuesday in the more than 200 towns in the Rio Doce river basin affected by the mining waste spill.

BHP's shares have slumped from Aus$23.50 at the time of the disaster to trade at Aus$20.33 on Thursday, although weak commodity prices have also contributed to the slide.