Twelve people have died and a quarter of all homes have been destroyed in the Florida Keys following Hurricane Irma, state officials say.

The fatalities, which take the storm's death toll to 61, were confirmed by emergency authorities on Tuesday as people across the region - an island chain stretching southwest from the tip of the Florida Peninsula into the Gulf of Mexico - began returning to their devastated neighbourhoods.

Rick Scott, governor of Florida, said there was a "lot of work to do" in the wake Sunday's Category 4 storm, which hit the largely evacuated area with winds of up to 215kph.

"We are bringing people back to a normal life as fast as possible ... everybody's going to come together, and we are going to get this state rebuilt," he said on Tuesday.

Initial damage assessments revealed that almost 65 percent of homes in the Florida Keys had sustained major damage, according to Brock Long, a Federal Emergency Management Agency administrator.

"Basically every house in the Keys was impacted," he said.

An estimated 5.8 million properties across the state were still without power yesterday, down from around 7.4 million on Monday.

Florida Power & Light Co, the state's largest utility company, said western parts of the territory might remain without electricity until September 22.

Limited service has now resumed at several of Florida's airports, including at Miami International.

Hurricane Irma, which was downgraded to a tropical depression on Monday, has drifted north into Alabama will likely continue to dissipate, the National Hurricane Center said yesterday.