A major power outage plunged much of Puerto Rico into darkness Tuesday, with the US island territory's electric utility saying restoration could take up to two days.

The "island-wide blackout" began at 5:30 AN (0930 GMT or 5:30 PM Philippine time), Luma Energy, which manages power transmission on the island, said in a social media statement.

It added that preliminary findings pointed to a fault on an underground cable as the cause of the outage — although the situation remains under investigation.

"We have already begun the restoration process for some customers, and the entire process will take 24 to 48 hours," Luma said.

The outage comes as the Caribbean island enters its high season for tourists visiting from the wintry US mainland, and just hours ahead of New Year's Eve celebrations.

Luma added in a subsequent statement that service had been restored in some areas, including a medical center and hospital in the capital San Juan, as well as the small island of Culebra.

Around noon (1600 GMT), about 15 percent of Luma's clients had service, according to a tracking site.

The company earlier noted that the incident left its 1.5 million or so customers out of service.

Buildings stood in the dark early Tuesday while traffic intersections were without stop lights, with only the headlights of vehicles lighting up the streets.

In a social media post, Puerto Rico Governor Pedro Pierluisi said officials were in communication with Luma and others on the major blackout affecting a large part of the island.

"We are demanding answers," he said, urging for the public to be informed on measures being taken to restore service throughout the island.

Luma said it would provide regular updates on the restoration process on social media.

Puerto Rico, a Spanish-speaking archipelago, came under US control in 1898 and has faced perennial infrastructure issues, exacerbated by the devastating Hurricane Maria in 2017.