Displaced Florida residents are heading back to their homes as a weakened Hurricane Irma advances inland, flooding several cities in the northeast of the state while millions of people remain without power.
Downgraded to a tropical storm early on Monday, Irma had ranked as one of the most powerful Atlantic hurricanes recorded. It cut power to millions of people and ripped roofs off homes as it hit a wide swath of Florida on Sunday and Monday.
Authorities said the storm had killed 38 people in the Caribbean and one in Florida, a man found dead in a pickup truck that had crashed into a tree in high winds on the Florida Keys over the weekend.
With sustained winds of up to 100 km/h, Irma had crossed into Georgia and was situated about 76 km northeast of the Florida state capital Tallahassee, the National Hurricane Center said.
High winds snapped power lines and left almost 6.5 million homes and businesses without power in Florida, Georgia, South Carolina and Alabama, state officials and utilities said. They said it could take weeks to complete repairs.
Miami International Airport, one of the busiest in the country, halted passenger flights through until at least Monday.
Police in Miami-Dade County said they had made 29 arrests for looting and burglary. Fort Lauderdale police said they had arrested 19 people for looting.
The storm did some $US20 billion ($A25 billion) to $US40 billion in damage to insured property as it tore through Florida, catastrophe modelling firm AIR Worldwide estimated.
Some 6.5 million people, about one-third of Florida's population, had been ordered to evacuate their homes ahead of Irma's arrival. More than 200,000 people sought refuge in about 700 shelters, according to state data.
President Donald Trump, attending a ceremony at the Pentagon on the 16th anniversary of the September 11 attacks, vowed a full response to Irma as well as ongoing federal support for victims of Hurricane Harvey, which flooded Texas.
"These are storms of catastrophic severity and we are marshalling the full resources of the federal government to help our fellow Americans," Trump said.
Florida's largest city, Miami, was spared the brunt of the storm but still suffered heavy flooding. Boats, including one named "Lucky Duck", had been driven ashore by the storm and utility crews were on the streets there clearing downed trees and power lines.