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.
Fiona packed winds as high as 215 km/h on Wednesday and was expected to strengthen as it moved north toward Bermuda although 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.
Eric Blake, acting branch chief for the NHC in Miami, said Bermuda would see high surf, storm surges, heavy rainfall and powerful winds even if Fiona kept on its current path and passed to the west of the island.
Bermuda will see the worst of the storm by late Thursday, the NHC said.
"Hopefully, the core of the storm will stay west but it could still jog east and hit Bermuda," Blake said, adding that the US east coast would experience large swells and rip currents as the storm churns toward Canada.
"This will be a big deal up there," he told Reuters, referring to Fiona's track toward Canada's Atlantic provinces of Nova Scotia as well as Newfoundland and Labrador.
In Puerto Rico, where 40 per cent of the island's 3.3 million residents were still without water and three-fourths were lacking power, authorities were trying to determine the scale of the destruction and start rebuilding.
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, Dr Maria Conte Miller, director of the Institute of Forensic Sciences, said in at roundtable on Tuesday.
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.
