At least two people were killed and many more were still missing on Sunday, three days after a dam burst at a mine in Brazil.

The Financial Times reported that the accident at the mine operated by Samarco (a company that is jointly owned by BHP Billiton and Vale) happened when weekend staff were carrying out work on one of the dams to increase its size when it burst, sweeping away workers and sending a mass of mine tailings into the town of Bento Rodrigues.

Rescue services said two people were confirmed to have died and the whereabouts of another 28 people — including 13 mine workers — remained unknown.

Brazilian media reported on Sunday that what is being called the “mud tsunami” had hit areas up to 100 km away from the mine.

Shares in BHP fell 5.6 per cent in Sydney on Monday to A$21.42, their lowest level in seven years.

BHP Billiton said its chief executive Andrew Mackenzie would travel to Brazil this week as the Anglo-Australian miner confronts the aftermath of the accident along with Vale, its Brazilian joint venture partner. BHP and Vale own 50 percent each of Samarco, the independently operated iron ore miner that controlled the project in Minas Gerais state where the accident occurred late on Thursday.

Mackenzie would go to meet Samarco’s response team as well as authorities and local community members “to understand first-hand the human, environmental and operational effects of the incident,” BHP said.

Authorities have placed on alert 15 cities that receive their water supplies from the Rio Doce, the river most affected by the dam burst, amid fears that the mud could cause additional flooding.

The accident is likely to become the largest for years involving either BHP, the world’s largest miner by market capitalisation, or Vale, the number one iron ore miner by output.

Jac Nasser, BHP’s chairman, said: “Words cannot describe the impact of this tragedy on the employees and contractors of Samarco, their families and the community.

“Our thoughts are with the people of Samarco, the affected community and with the people of Brazil.”

Samarco would be provided “with all the assistance necessary to support the response effort”, BHP said. Mr Mackenzie said BHP staff had been “deeply affected”.

Samarco said 253 people, or 70 families, were placed into hotels and guesthouses in the region. Hundreds of others were taken to a sports center in a nearby city. In total, 588 people have been relocated.

“The expansion of the mud on the Rio Doce is constantly being monitored by the company,” Samarco said.