Puerto Rico has been plunged into darkness on New Year's Eve by a nearly island-wide blackout.

More than 1.2 million out of almost 1.5 million customers have no electricity, said Luma Energy, the island's main power distributor.

Fully restoring service could take between 24-48 hours, the company said.

The blackout prompted renewed calls to address the unincorporated US territory's power issues, which have persisted since Hurricane Maria in 2017.

Power was restored by Tuesday afternoon to some areas, as well as San Juan's municipal hospital, Luma said.

Nearly 200,000 clients had power by late afternoon, Luma spokesman Hugo Sorrentini SAID.

A failure in an electric line at one of the main power plants, Costa Sur, caused cascading issues across the island, Mr Sorrentini said.

US President Joe Biden has been briefed on the situation and Energy Secretary Jennifer Granholm has spoken with Puerto Rico's governor to offer assistance, the White House said.

"We can't keep relying on an energy system that fails our people," Jenniffer González-Colon, Puerto Rico's current US congressional representative and the incoming governor of Puerto Rico, wrote on X.

"Today's blackout and the uncertainty around restoration continue to impact our economy and quality of life," she said.

On Facebook, the current governor, Pedro Pierluisi, demanded answers and solutions from the two main power companies, Luma and Genera.

Hundreds of thousands of residents at a time have been affected by power outages this year.

A June outage left about 350,000 customers without power as temperatures climbed, and more than 700,000 customers lost electricity after Hurricane Ernesto in August.

As they awoke to another day without power, Puerto Ricans expressed frustration.

"They're part of my everyday life," Enid Núñez, 49, said of the outages to the Associated Press.

Puerto Rico's power grid was strained even before Hurricane Maria devastated the island.

US government funding helped shore up the grid, facilitate recovery projects from other natural disasters, and make other important infrastructure improvements.

But the implementation has been incomplete due to a variety of factors, such as issues starting construction and the Federal Emergency Management Agency's requirements to authorise the use of some of the funds, according to a February 2024 report from the US Government Accountability Office.

"Inexcusably the power grid has still not recovered from damage in Hurricane Maria," Mark Levine, New York City's Manhattan borough president, wrote on X.

New York City is home to the largest Puerto Rican community in the mainland US.