BHP said there was at least one confirmed fatality with 13 members of the workforce missing, as well as at least 15 people from local communities.

“BHP Billiton’s chief executive officer, Andrew Mackenzie, will go to Brazil this week to understand first-hand the human, environmental and operational impacts of the incident,” the company said this morning.

“Meanwhile, BHP Billiton has offered its full support to help the immediate rescue efforts and to assist with the investigation.”

The company said its immediate priority was the welfare of the workforce and local communities, but it would work with operating company Samarco to determine the cause of the incident.

The Samarco operations include a three-tiered tailings dam. The Fundao dam failed, impacting the downstream Santarem dam.

“This resulted in a significant release of mine tailings, flooding the community of Bento Rodrigues and impacting other communities downstream,” BHP said.

“The third dam in the complex, the Germano dam, is being monitored by Samarco.”

Brazil’s National Department of Mineral Production there was a small earthquake, measuring around 2.5 on the Richter scale, in the region about one before the accident but it is unknown if that played a part.
 
 Samarco is a 50:50 joint venture between BHP and Vale.

Vale said it was deeply saddened by the incident and was offering its support.

"We would like to express our solidarity to all affected by this accident at Samarco's tailings dams in the state of Minas Gerais,” Vale CEO and president Murilo Ferreira said.

“We will do everything in our power to offer all the necessary support to Samarco and the authorities at this sad time for employees, their family members and the neighbouring communities.”

Brazilian President Dilma Rousseff declared a state of emergency and said it was important to establish the “causes and responsibilities” of the accident.

Samarco said overnight that it was taking action to mitigate the environmental impacts and had started collecting water samples.

The operations produce up to 30.5 million tonnes per annum of pellets and contributed around 3% to BHP’s underlying earnings in the 2015 financial year.

BHP said its full-year iron ore guidance was under review.

BHP and Vale shares each lost 5.7% in London and New York respectively on Friday.
 
Shares in BHP dropped another 3.7% in Australia this morning to $A21.85.