Irma, which was lashing the Florida Keys Sunday morning as a Category 4 hurricane, has already claimed three lives in Florida, officials said.
A man died Saturday after the truck he was driving crashed into a tree in Monroe County, which includes the islands of the Florida Keys. He apparently lost control of the vehicle due to a strong wind gust.
Local authorities said the truck was carrying a generator.
The sheriff of Monroe County, Rick Ramsay, said in a statement that the man was found dead Saturday afternoon in Marathon, in the middle of the Florida Keys, and that authorities had to wait for the winds to decrease in intensity before they could recover the body.
"We did not want to leave this man?s body out there and I saw a lull in the storm that would allow us to go out there and get it safely,? Ramsay said.
Two other people died Sunday morning in a car accident in Central Florida amid heavy rains from Irma, whose outer bands began affecting the state on Saturday.
The Florida Highway Patrol said a head-on collision occurred as weather conditions began to deteriorate due to the hurricane, although the crash is still under investigation.
Before making landfall Sunday morning in the Florida Keys, Irma had left at least 25 dead and caused severe damage in the Lesser Antilles and other islands of the Caribbean.
The Miami-based National Hurricane Center said in a special bulletin that the center of Hurricane Irma made landfall in the Florida Keys at 9:10 am (1310 GMT) with maximum sustained winds of 215 km/h (130 mph).
It said Cudjoe Key took a direct hit from Irma's eye wall.
Florida authorities and United States President Donald Trump have warned people in the state to seek shelter and abandon areas under a mandatory evacuation order.
A total of 6.3 million people - or around a third of the state's population - have been ordered to evacuate.
The powerful hurricane has thus far left more than 800,000 houses and offices in the state without electricity, authorities said.
In its 11 am public advisory, the NHC said the hurricane should move over the Lower Florida Keys shortly, and then move near or over the west coast of the Florida Peninsula later Sunday through Sunday night.
The storm has started to move north after having maintained a westward movement until shortly before leaving the coast of Cuba.
Along with the winds, heavy downpours pose significant danger and could leave rain accumulations of up to 25 inches in the Florida Keys and 20 inches in the southern part of the Florida peninsula.
The storm surge also poses a deadly threat, with water expected to reach 3-4.5 meters (10-15 feet) above ground in a large swath of Florida's southwest coast if peak surge occurs at the time of high tide.
The storm surge could flood Florida west coast cities such as Naples, Fort Myers and Marco Island.