Around 5.8 million people are without power after Hurricane Irma has hit mainland Florida, just south of Naples, amid warnings of storm surges as high as 15ft (4.5m).
“As little as six inches of moving water can knock you down,” said the state’s governor Rick Scott following the downgrade. “Stay inside. Stay safe.”
Describing the hurricane as “some big monster”, President Donald Trump approved a major disaster declaration and emergency federal aid for Florida.
The US National Hurricane Centre said Irma is expected to weaken to a tropical depression by Tuesday afternoon.