Argentina's state news agency says that most of the nation's grid is back up and running after a massive blackout that left tens of millions of people without electricity.

Telam said Sunday that Argentina had restored power to 90 percent of the South American country. Power also has been restored to most of Uruguay's 3 million people.

The massive blackout had left tens of millions of people without electricity in Argentina, Uruguay and Paraguay on Sunday in what the Argentine president called an "unprecedented" failure in the countries' power grid.

Authorities were working frantically to restore power, but 12 hours after the country went dark, more than a quarter of Argentina's 44 million people had still been without power and the cause of the outage remained unclear.