A power failure has blacked out Brazil’s two largest cities and other parts of the country for more than two hours, after a hydroelectric dam suddenly went offline.

Neighbouring Paraguay also lost power, but for only about 20 minutes, reports local media.

The Itaipu dam straddling the two nations’ borders stopped producing 17,000MW of power, resulting in outages in Rio de Janeiro, Sao Paulo and at least several other big Brazilian cities, Brazil’s mines and energy minister, Edison Lobão, announced.

He said outages hit nine of the 27 states in a country that is populated by more than 90 million people.

The cause of the failure has not been determined, but Lobão said strong storms that uprooted trees near the dam just before it went offline could be to blame.

Rio is believed to have been the hardest hit city.

Utility companies that provide electricity for Rio and Sao Paulo have not said why the power went off or when it would be restored, according to Agencia Brasil.