The centre of Hurricane Irma has made landfall in Florida Keys, the US National Hurricane Centre has said.
The northern eyewall of the storm reached the island chain earlier.
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The storm had maximum sustained winds of 130mph. The National Weather Service reported wind gusts of 90 mph (145 kph) near its Key West office.
After hitting the Florida Keys, Irma was forecast to move up the state's Gulf Coast later on Sunday.
The first death on mainland US has been reported after a man crashed his truck.
Officials have warned residents to stay inside, and be aware that fallen live wires and debris in the street, even once the storm is over, could be "killers".
Hundreds of thousands of homes and businesses are out of power, according to Florida's largest power company.
While more than six million people were ordered to evacuate, others said it was near impossible due to traffic jams, expensive flights and empty petrol stations.
