GUSAU, Nigeria (BBC) -- Some 40 people are feared dead in northern Nigeria after a dam collapsed. About 500 houses were swept away by the waters near Gusau, the capital of Zamfara state.

The state governor told the BBC that water behind the dam had reached a critical level following torrential rain, forcing the structure to burst.

Staff at the dam had been unable to open the floodgates, Nigerian media reports. State authorities have put up temporary shelters to house survivors.

Governor Ahmed Yarima called on the Nigerian federal government to provide aid.

He told the BBC that a tidal surge engulfed the area.

"The body of water was just like the pictures of tsunami that we have seen," he said. "It had enough force and speed to sweep people off their feet and into the river."

The dam was the main source of water supply to Gusau.