Downgraded to a tropical storm early on Monday, Irma had ranked as one of the most powerful Atlantic hurricanes on record before barreling into the Florida Keys on Sunday and plowing northward along the Gulf Coast and moving inland to wreak havoc across a wide swath of the third-most populous US state.

Still, the scope of damage in Florida and neighboring states paled in comparison with the utter devastation left by Irma as a rare Category 5 hurricane in parts of the Caribbean, where the storm killed nearly 40 people - at least 10 of them in Cuba - before turning its fury on Florida.

Especially hard hit in the United States was the resort archipelago of the Keys, extending into the Gulf of Mexico from the tip of Florida's peninsula and connected to the mainland by a single, narrow highway, Governor Rick Scott told a news conference on Monday.

"There's devastation," he said, adding that virtually every mobile-home park on the island chain was left upended. "It's horrible what we saw."

While some evacuees from the Keys expressed anger at authorities refusing to allow them to return to their homes on Monday, the US Defense Department said as many as 10,000 residents who had stayed put on the island may now be stranded and in need of evacuation.

Monroe County fire officials said later they would reopen road access on Tuesday morning at 7 a.m. EDT (1100 GMT) 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.

No timetable for reopening the remainder of the Keys was given.

In Miami, which escaped the worst of Irma's winds but experienced heavy flooding, residents in the city's Little Haiti neighborhood returned to the wreckage of trailer homes that were shredded by the storm.

"I wanted to cry, but this is what it is, this is life," Melida Hernandez, 67, who had ridden out the storm at a nearby church, said as she gazed at the ruins of her dwelling, split in two by a fallen tree.