MIAMI/SAN JUAN, P.R.,: Hurricane Irma has claimed nine more lives, including six patients at a Florida nursing home that lost power, authorities said on Wednesday, as more than four million people in four states struggled without electricity and residents slowly returned to the devastated Florida Keys.

Categorised as one of the most powerful Atlantic storms on record when it rampaged through the Caribbean, Irma has killed at least 70 people, officials said.

At least 26 people died in Florida and nearby US states, and destruction was widespread in the Keys, where Irma made initial US landfall on Sunday and became the second major hurricane to strike the mainland this season.

Police opened a criminal investigation at the Rehabilitation Center of Hollywood Hills, where two elderly residents were found dead at the facility and four later died at a hospital, officials said.

Some residents were evacuated early Sunday morning and some woke up feeling sick at the centre, which had been without air conditioning, Broward County Mayor Barbara Sharief told reporters on Wednesday.

“The building has been sealed off and we are conducting a criminal investigation inside,” Hollywood Police Chief Thomas Sanchez told reporters on Wednesday.

Prolonged power outages remains a risk for the elderly through the region, Sharief said.

“Elderly people and our population of children are very vulnerable, so they’re not out of the woods yet,” Sharief said.

“We need to try to restore power and try to get those people to a safe and comfortable place.” Some 4.3 million homes and businesses, or about nine million people, were without power at midday on Wednesday in Florida and nearby states.

Authorities continued to assess damage in the Florida Keys, where federal officials estimated that 25 per cent of homes were destroyed and 65 per cent suffered major damage when Irma barrelled ashore on Sunday as a Category 4 hurricane with sustained winds of up to 215 km per hour.

Most residents had left by then and police have barred re-entry to most of the Keys to allow more time to restore electricity and medical service and bring water, food and fuel.

Some 10,000 Keys residents stayed put when the storm hit and may ultimately need to be evacuated, according to officials.

“I don’t have a house. I don’t have a job. I have nothing,” said Mercedes Lopez, 50, whose family fled north from the Keys town of Marathon on Friday and rode out the storm at an Orlando hotel, only to learn their home was destroyed, along with the gasoline station where Lopez worked.

Federal Emergency Management Agency administrator Brock Long warned that life on the Keys would remain tough.

“There are numerous bridges of support that have to be inspected before we can put commodities down the roadway to support citizens,” Brock told CNN. “Communication is lacking in many portions of Monroe County. Citizens are frustrated about not being able to get the support they need right now. That’s exactly why we asked them to leave.” Three people died of carbon monoxide poisoning, including children aged 13 and 16, after a portable gas generator was left running inside an Orlando-area house, the Orange County Sheriff’s office said on Wednesday. Four others from the house were hospitalised.

President Donald Trump is due to visit the region on Thursday.

Irma wreaked total devastation in parts of the Caribbean, where at least 43 people have died.

People who fled their homes in hard-hit islands including St. Martin and the US Virgin Islands that were all but cut off from the world for days arrived in San Juan late Tuesday.

Michael Benson, 65, of St. John in the US Virgin Islands, said he lost everything.

“My house, my business, both my vehicles, everything is gone,” said Benson, who was stopping in San Juan before continuing to Boston to seek refuge with his wife’s brother.

“But we have life. We rode out that horrible storm in a shower that I had reinforced after Hurricane Marilyn,” Benson added. “I told the man (who installed the shower), I told him, ‘If the hurricane takes the rest of my house, I want this shower sticking up out of that slab like the last tooth in the mouth of a bum. And sure enough that’s what’s left.” Florida’s largest utility, Florida Power & Light Co, said western parts of Florida might be without electricity until September 22.

Irma hit the United States about two weeks after Hurricane Harvey ploughed into Houston, killing about 60 and causing some $180 billion (Dh661 billion) in damage, mostly from flooding.