(CNN) A "massive failure" in an electrical interconnection system left Argentina, Paraguay and Uruguay completely without power, and for many customers, restoring electricity would take all day, a utility distributor said Sunday.

Parts of Chile and southern Brazil experienced outages as well, said Edesur, the Buenos Aires-based company. Chile's system was running normally again Sunday afternoon, CNN Chile reported.

The company later posted an updated statement removing Uruguay and Paraguay from the list of countries that were entirely without electricity, but it isn't clear how many residents in those two countries have had their power restored. Power was only partially restored in Uruguay, the country's energy authority said.

Power had been restored to more than half of the country by Sunday evening, Gustavo Lopetegui, Argentina's energy secretary, said in a press conference.

Lopetegui said that experts are trying to determine the cause of the unprecedented blackout. As of now, no explanation for the widespread power failure has been identified.

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