Cuba says it was generating only enough electricity to cover about one-sixth of peak demand late on Wednesday, hours after its national grid collapsed leaving millions without power.

The National Electric Union (UNE) said it was producing 533 megawatts of electricity by evening, still just a fraction of typical dinnertime demand of between 3,000 and 3,200 megawatts, leaving most Cubans in the dark as night fell across the Caribbean island.

Earlier, the communist-run government said it would prioritize returning power to hospitals and water pumping facilities. Schools and non-essential government services were closed until further notice.

Lights flickered on across parts of the capital Havana late on Wednesday. The local electric company said more than 260,000 clients had seen power restored.

It was the latest in a string of countrywide blackouts of Cuba's antiquated and increasingly frail power generation system. This year, Cuba's grid fell into near-total disarray, stressed by fuel shortages, natural disaster and economic crisis.

Dwindling oil imports from Venezuela, Russia and Mexico pushed the island's obsolete and struggling oil-fired power plants into full crisis several months ago.

Hours-long rolling blackouts and severe shortages of food, medicine and water have made life increasingly hard for many Cubans, who in recent years have fled the island in record-breaking numbers.

Cuba blames U.S. sanctions, which complicate financial transactions and the purchase of fuel, for the crisis.