A blackout spreading across nine states of southern and central Brazil was caused by a failure at a hydroelectric dam, the country's energy minister said.

The power outage Tuesday night affected 40 million people and left about 800 cities - including Sao Paulo, Rio de Janeiro, Belo Horizonte and Brasilia - in the dark.

Energy Minister Edison Lobao speculated that a storm had disconnected power transmission lines at the Itaipu dam, which provides more than 20 percent of Brazil's energy.