A massive blackout across the southern half of Brazil and all of Paraguay this week was caused by a fierce storm shorting transmission lines from a huge binational power plant, Brazil's government said late Wednesday.

But the explanation, advanced by Energy Minister Edison Lobao after consultations with national grid officials, ran counter to information given by Brazilian and Paraguayan electricity companies.

Furnas, the state-run company supplying electricity to southern Brazil, said no damage had been found to any of the five high-tension lines feeding power out of the Itaipu hydro-electric plant straddling the Brazil-Paraguay border.

And a spokesman from ANDES, Paraguay's state-run energy firm, told AFP that while three of Itaipu's five lines had indeed gone off-line, the weather explanation was a "pretext" because the plant had continued operating.

The outage, which hit at 10:15 pm Tuesday (0015 GMT Wednesday) and lasted more than three hours, left an estimated 70 million people ? more than a third of Brazil's 190-million-strong population ? without electricity, including the major cities of Sao Paulo and Rio de Janeiro.

It imperiled traffic by leaving intersections without lights, stalled subway trains and elevators, and prompted off-duty police to be called up out of fear of a surge in nocturnal crime.

Lobao told a news conference that the consensus between grid operators and his ministry was that a storm must have somehow knocked out the lines running from Itaipu.

"We all arrived at the conclusion that what happened was the result of atmospheric discharges, very strong rain and wind," he said.

A domino effect meant 40 percent of the national energy supply went offline overnight, he said.

"Everything is completely working now," he reassured.

Doubts remained however over his explanation, given lack of concrete evidence and contradictory earlier information.

The "national interlinked grid is operating normally and no damage has been identified in its circuits and transmission towers," Furans said in its statement.

"Any diagnostic at this time is purely speculative," it cautioned.

The state ANDES electricity firm in Paraguay, which suffered a 30-minute outage across its entire small territory, confirmed Itaipu's transmission lines were at the source of the problem but expressed skepticism at the weather explanation.

"It happened as a consequence of a failure of three high-tension transmission lines of 750 000 volts each in the Sao Paulo region," a spokesman, Jacinto Bernal, told AFP.

"They should drop the pretext of attributing the problem to bad weather," he said. "Itaipu was able to keep producing but wasn't on-line."

Brazil was defensive over the blackout, especially given international media attention that raised questions over its preparedness to host the 2016 Olympic Games in Rio.

President Luiz Inacio Lula da Silva, speaking before his energy minister's news conference, sought to dismiss doubts over Brazil's energy generation.

"There was no lack of energy production. Energy continues to be produced. We had a problem in the transmission line," he told reporters while hosting a visit to Brasilia by Israeli President Shimon Peres.

The blackout occurred two nights after the US television network CBS broadcast a report in which unidentified former US national security officials claimed massive power outages in Brazil in 2005 and 2007 were caused by cyber hackers attacking control systems.

Although Brazilian media and officials were extremely skeptical of that unconfirmed assessment, the US channel said those incidents should serve as a wake-up call to the United States, which could see its own power supplies hit by computer sabotage.