After making landfall in Puerto Rico on Sunday, Fiona caused devastating flooding and landslides on the island.
Over the next two days, the storm gathered steam as it barrelled into the Dominican Republic and the Turks and Caicos Islands, where it left most people without power and up to eight people dead.
Fiona packed winds as high as 215km/h on Wednesday and was expected to strengthen as it moved north toward Bermuda, though no direct hit is forecast for the British territory, the US National Hurricane Center (NHC) said.
Fiona could reach Canada's Atlantic coast on Friday.
In Puerto Rico, where 40 per cent of the island's 3.3 million residents were still without water and three-quarters were lacking power, authorities were trying to determine the scale of the destruction and start rebuilding.
US President Joe Biden on Wednesday approved a disaster declaration for Puerto Rico, which makes federal funding available for individuals affected by the storm, the White House said in a statement.
Fiona might have caused at least eight deaths, including that of a sick four-month-old infant whose mother struggled to get to the hospital due to blocked roads. The deaths are under investigation.
The US Federal Emergency Management Agency has so far attributed four deaths to the storm in Puerto Rico. A fifth person was killed in Guadeloupe earlier in the week.
For many Puerto Rico residents, the memory of Hurricane Maria in 2017 is still fresh. Some 3000 people died in that Category 5 storm, which left the entire island without electricity for a week.
An estimated 1.07 million homes and businesses remained without power in Puerto Rico by midday on Wednesday, and full restoration to all 1.5 million customers could take several days.
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