* At least 54 killed by storm, including at least 11 in US* Millions still without power
* Residents return to parts of Florida Keys
* Authorities working to restore basic services
* Irma could be long-term boost to economy - Mnuchin
Residents returned on Tuesday to parts of the Florida Keys archipelago that was hammered by Hurricane Irma's high winds and storm surge, while the death toll rose in the second major hurricane to hit the United States this year.
Irma, which had rampaged through the Caribbean as one of the most powerful Atlantic hurricanes on record, was downgraded to a tropical depression on Monday and would likely dissipate from Tuesday evening, the National Hurricane Center said.
At its peak the storm prompted evacuation orders for 6.5mn people in Florida, the largest evacuation in modern US history.
Irma killed 43 people in the Caribbean and at least 11 in Florida, Georgia and South Carolina.
A local Florida official said there had been more deaths yet to be reported, particularly on the Florida Keys, where Irma arrived on Key Cudjoe as a Category 4 hurricane with sustained winds of up to 130 miles per hour (215 km per hour) on Sunday.
Local authorities told around 90,000 residents of Miami Beach and people from some parts of the Keys they could go home but warned it might not be prudent to remain there.
"This is going to be a frustrating event. It's going to take some time to let people back into their homes particularly in the Florida Keys," Brock Long, administrator of the Federal Emergency Management Agency, told a news conference.
Millions of people were still without power in Florida.
Many areas flooded
The city of Jacksonville, in Florida's northeast, was recovering from heavy flooding.
"There are so many areas that you would never have thought would have flooded that have flooded," Florida Governor Rick Scott told reporters after a helicopter tour of the area.
Irma devastated several Caribbean islands en route to Florida. It destroyed about one-third of the buildings on the Dutch-ruled portion of St. Martin island, the Dutch Red Cross said on Tuesday.
The storm was the second major hurricane to hit the United States in a little more than two weeks. Hurricane Harvey plowed into Houston late last month, killing about 60 and wreaking some $180bn in damage, largely through flooding.
Monroe County Commissioner Heather Carruthers said on Monday that people had been killed in the Keys, which have nearly 80,000 permanent residents, but she did not have a count on how many.
The US aircraft carrier Abraham Lincoln was off Florida's east coast and two amphibious assault ships were en route to help in the Keys.
Monroe County opened road access on Tuesday morning for residents and business owners from Key Largo, the main island at the upper end of the chain, as well as the towns of Tavernier and Islamorada farther to the south, fire officials said.
No timetable was given for reopening the remainder of the Keys, which are linked by a series of causeways and bridges down to Key West, a popular tourist spot on the southern tip of Florida.
Deaths during cleanup
Several major airports in Florida that halted passenger operations due to Irma began limited service on Tuesday, including Miami International, one of the busiest US airports.
Utility companies reported some 6.9mn homes and businesses were without electricity in Florida and neighboring states and said it could take weeks to fully restore service.
Insured property losses in Florida from Irma were expected to run from $20bn to $40bn, catastrophe modeling firm AIR Worldwide estimated.
US Treasury Secretary Steven Mnuchin told an investor conference in New York that the storm would ultimately boost the economy by sparking rebuilding.
"There clearly is going to be an impact on GDP in the short run, we will make it up in the long run," Mnuchin said. "As we rebuild, that will help GDP. It's too early to tell what the exact estimates will be, but I think it won't have a bad impact on the economy."
Several of the deaths caused by Irma came as people started cleaning up and making repairs.
A 55-year-old man died Monday in Tampa, Florida, while using a chainsaw in a tree during storm cleanup, the Hillsborough County Sheriff's Office said.
A man died in Worth County, Georgia, on Monday while repairing the roof of a shed during sustained winds of 42 mph (67 kph) with gusts up to 70 mph (112 kph), a National Weather Service report said.
A man was found dead in Winter Garden, Florida, after being electrocuted by a downed power line, local police said.
One man in South Carolina was killed by a falling tree limb and another died in a traffic accident, officials said.
The National Hurricane Center was monitoring another hurricane, Jose, which was spinning in the Atlantic about 700 miles (1,130 km) west of Florida. About 2-1/2 months remain in the Atlantic hurricane season, which starts in June.