Cuba's power-grid operator said it had restored electricity to parts of the capital Havana on Monday following a fourth major grid failure in 48 hours, while Tropical Storm Oscar lashed the island's eastern end.

Strong winds and heavy rain uprooted trees and scattered downed power lines in places, complicating recovery efforts.

Energy and Mines Minister Vicente de la O Levy said technicians were working carefully to avoid another electrical collapse given "complex" circumstances.

"The last thing we want is that, as a consequence of a fallen power line, we suffer another collapse of the system," de la O Levy said.

Cuban energy officials said they were providing to the grid around 700 megawatts, or one-fourth of a typical day's demand, by mid-morning. Authorities said they had restored power to 56 per cent of Havana by midday.

WATCH | Cuban officials says U.S. embargo contributed to shortages: 

Millions in Cuba still without power, days after electricity grid collapse

Millions of Cubans are still without electricity, three days after the national power grid failed. Cuban officials blame the U.S. economic embargo for fuel shortages and a lack of spare parts that could ease the crisis.

Officials said they had also restored electricity to parts of some outlying provinces on the Caribbean's largest island.

Cuba's Communist-run government closed schools and non-essential industry through Wednesday as work continued.

Cuba's national electrical grid first crashed on Friday, before Oscar's arrival, after the island's largest power plant shut down, leaving 10 million people without electricity.

The grid has fully or partially failed three times since, underscoring the precarious state of the country's infrastructure and putting many Cubans on edge, as they already suffer from dire shortages of food, fuel and medicine.

Havana was largely quiet overnight with the entire city in blackout. But a Reuters witness saw several scattered protests in poor, outlying neighbourhoods, as well as residents banging pots in frustration over the blackouts and food and water shortages.

President Miguel Diaz-Canel spoke to the nation on national television late on Sunday, telling Cubans to air their grievances with discipline and civility.

"We are not going to accept nor allow anyone to act with vandalism and much less to alter the tranquility of our people," Diaz-Canel said.

Cubans have endured prolonged blackouts of 10 to 20 hours a day across much of the country for months, spoiling precious food stocks and complicating access to fuel and water.

The government and independent experts say the grid, long near collapse, has reached a critical point as obsolete infrastructure deteriorates and fuel runs short.

Cuba blames the U.S. trade embargo, as well as sanctions instituted by former U.S. President Donald Trump for difficulties in acquiring fuel and spare parts to operate and maintain its oil-fired plants.

The U.S. has denied any role in the grid failures.

Cuban allies Russia, Mexico and Venezuela have all slashed exports to the island in recent months.