Hurricane Dorian has strengthened into a category 3 system and could be a significant threat to Florida and the northwestern Bahamas, the US National Hurricane Centre warns.

Dorian was about 715 kilometres east of the northwestern Bahamas with maximum sustained winds of 185km/h, the centre said on Saturday morning (Australian time).

Additional strengthening is forecast, and Dorian is anticipated to remain an extremely dangerous major hurricane while it moves near the northwestern Bahamas and approaches the Florida peninsula into early next week, the NHC said.

Florida’s governor on Friday urged residents to stock up on at least a week’s worth of food, water and medicine and to prepare to lose power and mobile phone service for days after Dorian makes landfall early next week.

The slow march and rising intensity of the storm has alarmed forecasters, who worry parts of Florida will be walloped by strong winds, a storm surge and heavy rain for an extended period.

“I think there’s a pretty high degree of certainty that this is going to be a major hurricane,” Florida Governor Ron DeSantis told a news conference from the state emergency operations centre in Tallahassee.

He said residents should prepare for a “multi-day event”.

Two thousand National Guard troops will have been mobilised for the hurricane by the end of Friday, with another 2000 joining them on Saturday, Florida National Guard Major General James Eifert said.

Florida is under a declaration of emergency. In neighbouring Georgia, Governor Brian Kemp has declared a state of emergency in 12 counties to assist with storm readiness, response and recovery.

“Almost everyone has left, or is leaving today,” said Pauline Powell, 26, who was on a family vacation at the Island Seas Resort in Grand Bahama when the storm started barrelling toward the area’s pristine beaches.

The Bahamas Ministry of Tourism and Aviation said hotels and resorts throughout that part of the island nation have activated hurricane response programs, which typically include boarding up beach-facing windows and encouraging guests to leave.

The Grand Bahama International Airport in Freeport will close on Friday night and will not open until September 3, the ministry said in a statement.

If, as expected, Dorian reaches category 4 strength over the weekend, its winds will blow at more than 210km/h.

Forecasters predicted it will be near the Florida peninsula late on Monday.

They expect a storm surge of as much as 3-4.5 metres above normal in some coastal waters of the Bahamas and Florida, and rainfall of about 15-30cm, with up to 46cm in some areas.