(CNN) Argentina's energy secretary said he does not believe a cyberattack caused a massive power outage that left tens of millions of people in Argentina, Paraguay and Uruguay in darkness for several hours on Sunday.

"At this moment we do not rule out any possibilities but ... a cyberattack is not within the preliminary alternatives being considered," Gustavo Lopetegui told reporters on Sunday.

Argentina's President Mauricio Macri called the power outage, which also affected parts of Chile and southern Brazil, "unprecedented" and announced an official investigation into the cause.

As of now, no explanation for the widespread power failure has been identified.

The blackout comes as concerns rise over governments' abilities to disrupt other nations' power systems. The New York Times reported on Saturday that the United States is escalating cyber attacks on Russia's electric power grid and has placed potentially crippling malware inside the Russian system.

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