The United States began evacuating coastal areas on Wednesday as Hurricane Matthew churned toward the Bahamas, after killing at least 14 people in the Caribbean in a maelstrom of wind, mud and water.

Haiti's presidential election, scheduled for Sunday, was postponed after Matthew blasted the impoverished nation, knocking out a key bridge providing access to the country's south where the storm made landfall.

Matthew, the worst storm to hit the Caribbean in nearly a decade, also forced the closure of airports in the Bahamas, which was girding for a hit as early as Wednesday evening.

All cruise ships have been re-routed from the popular tourist destination, the government said.

"Seriously consider now moving to higher grounds. Natural phenomena can be violently unpredictable," Bahamian Prime Minister Perry Christie said, addressing southern coastal communities.

US President Barack Obama warned Americans in the storm's path -- including more than a million people told to evacuate from vulnerable coastal areas -- to take the threat seriously.

"We hope for the best, but we want to prepare for the worst," he said.

The storm slammed into Haiti and Cuba as a Category Four hurricane on Tuesday but has since been downgraded to three, on a scale of five, by the US National Hurricane Center (NHC).

Its winds were howling at 115 miles per hour (185 kilometers per hour).

- No relief -

The NHC said Matthew's recent weakening would be "short-lived," forecasting that the storm would intensify late Wednesday into Thursday.

In the southeastern US state of Florida, a frequent target of hurricanes, residents were calm -- with Matthew on track to approach the coast Thursday evening.

"I cannot emphasize it enough that everyone in our state must prepare now for a direct hit," said Governor Rick Scott, as evacuation orders were issued -- some voluntary, some mandatory -- across different parts of the state.

Schools and universities closed for the rest of the week, authorities were distributing sandbags for doorways, and store shelves were bare.

"We're not really afraid, Florida has been through this thing for years," said Rick Basel, 63, a retiree loading his car with food and water to last three or four days.

Further north in South Carolina, Governor Nikki Haley ordered the evacuation of several coastal counties, affecting more than one million people.

The order included the historic city of Charleston, a tourist magnet boasting cobblestone streets lined with graceful antebellum homes.

News footage showed bumper to bumper jams on highways and tensions were high as some gas stations ran out of fuel.

"Please do not call 911 because a gas station is out of gas. Remember to be patient with one another," the police department in Mount Pleasant, South Carolina said on Twitter.

The state's emergency authority warned against price gouging and reminded would-be profiteers that such practices could be punishable by a $1,000 fine, 30 days in jail, or both.

The US Air Force was moving aircraft from bases in Florida, Virginia and North Carolina ahead of the storm. Parts of Joint Base Charleston in South Carolina were also shuttering.

- Decimated -

As Matthew barreled northwest, Haiti and Cuba began the grim task of assessing the storm's toll.

Matthew's overall death toll stood at 14 -- 10 in Haiti and four in the Dominican Republic -- but looked certain to climb.

The United Nations office for coordinating humanitarian affairs said half of Haiti's population of 11 million was expected to be affected in one way or another.

Across the region more than 600,000 people are in emergency shelters, more than half of them in Haiti where thousands have lived in tents since the massive earthquake in 2010, the UN said.

At least 350,000 people in Haiti need immediate assistance, Secretary General Ban Ki-moon's office said, quoting the Haitian government.

The US military said Wednesday it is sending helicopters, an aircraft carrier and troops to provide humanitarian assistance to Haiti -- which had not been hit by a Category Four storm in 52 years.

In Cuba, where some 1.3 million people were evacuated, there were no reported fatalities but four cities in the east were cut off because roads were blocked by large chunks of rock hurled by the storm.

Officials reported flooding and waves up to five meters (16 feet) in eastern coastal villages.

Matthew also decimated Baracoa, the first Spanish settlement in Cuba.

"There's nothing left of Baracoa. Just debris and remains," said resident Quirenia Perez, 35, speaking to AFP by cell phone after losing her roof, electricity and landline in the storm.

She added: "The big colonial houses in the city center, which were so pretty, are destroyed."