"It was a real tsunami," Wellington Dias, the state governor, said after he visited the area.

Globo TV, a local television channel, said the rupture sent 50bn litres of water pouring out of the reservoir, causing flooding that stretched 100km downriver from Cocal.

Flooding and mudslides in northeast Brazil over the past few weeks have left 57 people dead.

The Brazilian government said weeks of rain and heavy flooding had displaced 400,000 people, about 95,000 of them in Piaui state.