Hurricane Ian has knocked out power across all of Cuba, leaving 11 million people without electricity, after it slammed into the island's western tip.
Cuba's Electric Union said power was initially knocked out for about one million people in the country's western provinces, but later the entire grid collapsed.
The hurricane made landfall as a Category 3 storm on the island's western end and devastated Pinar del Rio province, destroying some of the country's most important tobacco farms.
Tens of thousands of people have been evacuated or had fled before Ian caused flooding and damaged houses.
The US National Hurricane Center said Cuba suffered "significant wind and storm surge impacts" when the hurricane struck with top sustained winds of 125mph.
The hurricane was expected to get even stronger over the warm Gulf of Mexico as it approaches the southwestern coast of Florida, where 2.5 million people have been ordered to leave.
Residents and holidaymakers have been warned not to be complacent amid fears the hurricane could be upgraded to Category 4, with 140mph winds expected to affect the whole state.
At an emergency briefing at the White House on Tuesday, Deanne Criswell, spokeswoman for the Federal Emergency Management Agency (FEMA), said: "The storm surge is going to be significant, and you put 20 inches of rain on top of that….
"Take this very seriously, do not underestimate the potential this storm can bring.
"We are talking about impacts in parts of Florida that haven't seen a major direct impact in nearly 100 years."
Ian is heading towards the Florida Keys, a popular holiday destination, made up of many islands, some less than a mile wide.
US President Joe Biden cancelled a planned trip on Tuesday and called mayors in three Florida cities to assure them federal support is ready to deploy food, fuel and shelter.
A total of 29 emergency shelters have already been set up by FEMA personnel sent to the state on Monday.
Ron DeSantis, the governor of Florida, has declared a state of emergency and urged residents to prepare.
He warned of "broad impacts throughout the state" and said the weather system brought with it a risk of a "dangerous storm surge, heavy rainfall, flash flooding, strong winds, hazardous sea, and isolated tornadic activity".
Flooding is predicted for much of the Florida peninsula midweek, and then heavy rainfall is possible for the southeast of the US later this week.