Buenos Aires: Argentina has not ruled out a cyber attack as the reason why most of its 44 million citizens and those of some of its South American neighbours were left in the dark for hours on Sunday.

The mysterious blackout hit Argentina, Uruguay and Paraguay, with some reports of power interruptions also in Chile and Brazil.

Cars drive through an unlit street during a blackout in Buenos Aires. Credit:AP

Argentina's state news agency said that most of the nation's electricity grid was back up and running late on Sunday.

Telam said power has been restored to 90 per cent of the country. Power also has been restored to most of Uruguay's 3 million people.