Governor Ron DeSantis has declared a state of emergency for all of Florida as Tropical Storm Ian gained strength over the Caribbean and was forecast to become a major hurricane soon days as it tracked toward the state.

DeSantis had initially issued the emergency order for two dozen counties on Friday. But on Saturday he expanded the warning to the entire state, urging residents to prepare for a storm that could lash large swaths of Florida.

“This storm has the potential to strengthen into a major hurricane and we encourage all Floridians to make their preparations,” DeSantis said in a statement.

“We are coordinating with all state and local government partners to track potential impacts of this storm.”

US President Joe Biden also declared an emergency for the state, authorising the Department of Homeland Security and the Federal Emergency Management Agency (FEMA) to coordinate disaster relief efforts and provide assistance to protect lives and property. The president postponed a scheduled September 27 trip to Florida due to the storm.

The National Hurricane Centre said Ian was forecast to strengthen before moving over western Cuba and toward the west coast of Florida and the Florida Panhandle by the middle of next week.

The agency said Floridians should have hurricane plans in place and advised residents to monitor updates of the storm's evolving path. Ian was expected to become a hurricane Sunday and a major hurricane as soon as late Monday.

The storm had top sustained winds of 50 mph (85 kph) Saturday night as it swirled about 395 miles (630 kilometres) southeast of Grand Cayman, in the Cayman Islands.

A hurricane warning was in effect for the island, and hurricane watches were issued for western Cuba. “Ian forecast to begin rapidly intensifying,” the hurricane Centre reported.

John Cangialosi, a senior hurricane specialist at the Miami-based centre said it wasn't yet clear exactly where Ian will hit hardest in Florida. He said the state's residents should begin preparing for the storm, including gathering supplies for potential power outages.

“Too soon to say if it's going to be a southeast Florida problem or a central Florida problem or just the entire state,” he said.

“So, at this point really the right message for those living in Florida is that you have to watch forecasts and get ready and prepare yourself for potential impact from this tropical system.”

In Pinellas Park, near Tampa, people were waiting in line at a Home Depot when it opened at 6 am, the Tampa Bay Times reported.

Manager Wendy Macrini said the store had sold 600 cases of water by the early afternoon and ran out of generators.

People also were buying up plywood to put over their windows: “Better to have it and not need it than to need it and not have it,” Matt Beaver, of Pinellas Park, told the Times.

The governor's declaration frees up emergency protective funding and activates members of the Florida National Guard, his office said.

His order stresses that there is risk for a storm surge, flooding, dangerous winds and other weather conditions throughout the state.

Elsewhere, powerful post-tropical cyclone Fiona crashed ashore early on Saturday in Nova Scotia in the Atlantic Canada region.

The storm washed houses into the sea, tore rooftops off others and knocked out power to the vast majority of two Canadian provinces with more than 500,000 customers affected at the storm's height.

Fiona had transformed from a hurricane into a post-tropical storm late Friday, but it still had hurricane-strength winds and brought drenching rains and huge waves. There was no confirmation of fatalities or injuries.