(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.
In a statement on its website, Edesur said a "collapse" in Argentina's government-operated interconnection system occurred around 7 a.m. (6 a.m. ET).
The outage "is the first generalized blackout that Argentina has had in its history," Edesur spokesman Alejandra Martínez told CNN affiliate TN.
Edesur is prioritizing any customer who depends on electricity for health reasons, it said, but because the power outage is so serious, anyone experiencing problems should go to a medical center.
A tweet posted just before 10 a.m. (9 a.m. ET) said the company had begun generating electricity in Buenos Aires and the surrounding area. It said restoring power in greater Buenos Aires would take several hours.
Lucas Rodriguez tweeted a video of the Argentine capital in darkness before dawn, saying he'd never seen anything like it.
"The funny part is that we don't have electricity, but we have internet in our phones," he told CNN.
Edesur had restored service to more than 2 million customers, roughly 80% of its clients, by 3 p.m., the company said.
Power was also restored to parts of western Uruguay that border Argentina, and to some regions in the south, including Montevideo, said National Administration of Power Plants and Electrical Transmissions, the country's energy authority.
The cause of the failure is under investigation, said Edesur, which has launched an "emergency operational plan" to deal with the situation.
Utility distributor Edenor, which controls 20% of the Argentine market, about 3 million customers, said a transmission system at Yacyretá Dam -- on the Paraná River near Ayolas, Paraguay -- failed "without human intervention," forcing an automatic shutdown.
Edesur added that the failure began in a transport connection between the dam and the Salto Grande power stations on Argentina's coast. The shutdown was a protective measure, it said.
Roughly 95% of Edenor's customers had had their power restored by 3 p.m., the company said.
The Argentine Interconnection System, which experienced the failure, handles the bulk of Argentina's electricity. It is one of two such systems in the country, the other being in the Patagonia region.
The utility won't be able to fully restore power to the region until the interconnection system is running normally again, which could take all day, it said.
The three countries experiencing total blackouts are home to a collective 55 million people.