A dam has burst at a BHP owned mining waste site in Brazil, killing at least 17 people and injuring more than 50.
"The number of missing is going to surpass 40 but that is not official," said Adao Severino Junior, fire chief in the city of Mariana in southeastern Minas Gerais state.
The village of Bento Rodrigues near the dam is practically buried in mud, the fire chief said.
"The situation is grim. It is dark. There is a lot of mud," Severino said.
Rescue operations will continue through the night, he added.
The iron ore mine is owned by Samarco, a joint venture between Brazil's Vale and Australia's BHP.
The company said in a statement it had not yet determined why the dam burst on Thursday or the extent of the disaster.
Civil defence authorities in Mariana said it was evacuating about 600 people to higher ground from the village of Bento Rodrigues, where television footage showed dozens of homes destroyed by the mudslide.
A car rested on top of a wall where the roof of a building had been ripped off.
They said the flood had also reached another village further down the hill, called Paracat de Baixo, and that inhabitants there were being evacuated.
The dam was holding tailings, a mining waste product of metal filings, water and occasionally chemicals.
It was located near the Gualaxo do Norte river, adding to fears of potential water contamination.
BHP said it was pressing Samarco for more information and expressed concern in a statement for the safety of employees and the nearby community.