Dorian intensifies as it bears down on the Bahamas en route to the southeastern coast of the US.

Hurricane Dorian gained fearsome new muscle as an “extremely dangerous” Category 4 storm, bearing down on the northwestern Bahamas on Saturday en route to Florida’s east coast.

Forecasters say Dorian, which had top sustained winds of 225 kilometres per hour on Friday night, will threaten the Florida peninsula late on Monday or early Tuesday.

The storm has alarmed forecasters who worry parts of Florida will be walloped by a storm surge and heavy rain for an extended period when it makes landfall.

The Miami-based National Hurricane Center said Dorian was expected to continue strengthening as it moved northwest near the Bahamas and approached Florida.

“This is big and is growing and it still has some time to get worse,” Julio Vasquez said at a Miami fast-food joint next to a gas station that had run out of fuel. “No one knows what can really happen. This is serious.”

Florida Governor Ron DeSantis urged residents to have at least a week’s worth of food, water and medicine, saying at a news conference in Tallahassee they should be prepared for a “multi-day event”. Florida is under a declaration of emergency.

About 4,000 National Guard troops would be mobilised for the hurricane, Florida National Guard Major-General James Eifert said.

Early Saturday, Dorian was centred 755km east of West Palm Beach. It was moving northwest at 17kph. Forecasters warned its slow movement meant Florida could face prolonged winds, storm surge and torrential rain.

Shoppers wait to buy supplies in advance of the arrival of Dorian in Kissimmee, Florida [Gregg Newton/Reuters]

Employees at Conchy Joe’s Seafood in Jensen Beach on Florida’s east coast boarded up windows and stripped bare the restaurant’s waterfront tiki bar before heading to their homes to make similar preparations.

“I’m planning on boarding up my house this evening,” said Jamison Weeks, the restaurant’s 38-year-old general manager. “The mood is a little tense, everybody’s a little nervous and just trying to prepare as best as possible.”

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Dorian is expected to bring a life-threatening storm surge that raises water levels by as much as three to 4.5 metres above normal levels in parts of the northwest Bahamas, the NHC said. Near the coast, the surge will be accompanied by large and destructive waves.

“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 towards 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 programmes, which typically include boarding up beach-facing windows and encouraging guests to leave.

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

‘Really worried’

Dorian began on Friday over the Atlantic as a Category 2 hurricane on the five-step Saffir-Simpson Wind Scale but strengthened to a Category 4.

“What’s concerning is it’s slowed down. We’re looking at a multiple-day event. We were hoping it would just barrel through and leave,” said Ben Malik, the mayor of Cocoa Beach, about 136km north of Port St Lucie.

“I’m really worried about the amount of rain we’ll be getting.”

Florida officials also were making sure all nursing homes and assisted living facilities had generators, and were checking with more than 100 facilities where information about generators was uncertain.

Store shelves are empty of bottled water as residents buy supplies [Marcus Lim/AP]

Officials from the Federal Emergency Management Agency, which was harshly criticised in 2005 for its slow response to Hurricane Katrina and its aftermath, briefed Congress on Friday on the steps they were taking to prepare for the storm.

President Donald Trump on Thursday said he cancelled a planned weekend trip to Poland so he could make sure resources were properly directed for the storm.

Dorian could churn across dozens of launchpads owned by NASA, the Air Force and companies such as Elon Musk’s SpaceX and Jeff Bezos’s Blue Origin.