Hurricane Dorian is gaining strength at it approaches Florida, with forecasters warning it could grow into a dangerous storm before it hits the east coast state.

The hurricane has moved out into open waters after hitting Puerto Rico and the US Virgin Islands, where it caused power cuts and flooding in places.

The US National Hurricane Centre said Dorian would probably strengthen into a dangerous Category 3 hurricane as it passes near or over the northern Bahamas on Saturday, before hitting Florida on Sunday.

The storm was a Category 1 hurricane when it swirled through the islands of the northeastern Caribbean on Wednesday.

Dorian caused an island-wide blackout in St Thomas and St John in the US Virgin Islands, and scattered power outages in St. Croix, government spokesman Richard Motta said.

Tropical storm Dorian expected to strengthen to hurricane Show all 7 1 /7 Tropical storm Dorian expected to strengthen to hurricane Tropical storm Dorian expected to strengthen to hurricane Shoppers preparing for the arrival of tropical storm Dorian stock up on supplies at a supermarket in Bridgetown, Barbados, on 25 August 2019. The US National Hurricane Centre warned the storm was expected to strengthen to a hurricane by the time it reached Puerto Rico on 28 August 2019. REUTERS/Nigel R Browne Tropical storm Dorian expected to strengthen to hurricane Residents board up a pharmacy in Bridgetown, Barbados, 26 August 2019. AP/Chris Brandis Tropical storm Dorian expected to strengthen to hurricane Storm clouds gather as tropical storm Dorian moves towards St Michael Parish, Barbados, 26 August 2019. AP/Chris Brandis Tropical storm Dorian expected to strengthen to hurricane Nearly empty shelves where bottled water is normally displayed at a supermarket in Cabo Rojo, Puerto Rico, 26 August 2019. REUTERS/Ricardo Arduengo Tropical storm Dorian expected to strengthen to hurricane Hotel employees remove sun loungers from a pool area in Gros Islet, St Lucia, 26 August 2019. REUTERS/Andrea de Silva Tropical storm Dorian expected to strengthen to hurricane Empty shelves that are normally filled with bread are seen empty at a supermarket in Gros Islet, St Lucia, 26 August 2019. REUTERS/Andrea de Silva Tropical storm Dorian expected to strengthen to hurricane Tropical storm Dorian is pictured off the coast of Venezuela in this NASA satellite photo, 26 August 2019. NASA/Handout via Reuters

The storm downed trees and at least one electric pole in St Thomas, he said, adding that there were no reports of major flooding. “We are grateful that it wasn’t a stronger storm,” Mr Motta said.

There were no reports of serious damage in the British Virgin Islands, where Governor Augustus Jaspert said crews were already clearing roads and inspecting infrastructure.

The US National Hurricane Centre said the storm’s maximum sustained winds had increased to 85 mph, warning that Dorian would strengthen and could hit anywhere from South Florida to South Carolina.

“This will be a large storm approaching the southeast,” said meteorologist Dennis Feltgen.

Puerto Rico residents board up their home ahead of Storm Dorian's arrival (Getty Images)

County governments along Florida’s east-central coast have distributed sandbags and many residents have rushed to warehouse retailers to load up on water, canned food and emergency supplies.

“All Floridians on the East Coast should have 7 days of supplies, prepare their homes & follow the track closely,” Governor Ron DeSantis said in a tweet. Later Wednesday, he declared a state of emergency for the counties in the storm’s path.

Puerto Rico seemed to be spared any heavy wind and rain, a huge relief to many on an island where blue tarps still cover some 30,000 homes nearly two years after Hurricane Maria.

The island’s 3.2 million inhabitants also depend on an unstable power grid that remains prone to outages since it was destroyed by Maria, a Category 4 storm.

Before the storm, Donald Trump sent a tweet assuring islanders that “FEMA and all others are ready, and will do a great job”.

He then added a jab at Puerto Rican officials who have accused his administration of a slow and inadequate response to Hurricane Maria, suggesting they should be grateful.

“When they do, let them know it, and give them a big Thank You – Not like last time. That includes from the incompetent Mayor of San Juan!”

The mayor, Carmen Yulin Cruz, responded on Twitter by saying Mr Trump needs to “calm down get out of the way and make way for those of us who are actually doing the work on the ground”.