The powerful Category 4 storm has already left a trail of destruction across the Caribbean, killing at least 11 people - and slammed into Haiti with howling 145mph winds.

Torrential rain demolished a key bridge in the impoverished nation, tore down phone lines and made major roads impassable, with the government warning it is difficult to determine how many Haitians have been injured or killed.

The peninsula battered most in Haiti has been cut off from the rest of the country, with floodwater running at shoulder height in some places, and reserves of drinking water running low.

"What we know is that many, many houses have been damaged. Some lost rooftops and they'll have to be replaced while others were totally destroyed," interior minister Francois Anick Joseph said.

Five people are known to have died in Haiti so far - including a man who drowned as he attempted to rescue a child who fell into a swollen river. The child was saved.

In the neighbouring Dominican Republic, four lost their lives. One person died in Colombia, and another in St Vincent and the Grenadines.

Although some residents in shanty towns in Haiti were evacuated to shelters, many refused - fearing their already limited possessions may be stolen.

The slow-moving storm - the worst in the region for nearly a decade - is now expected to move up the length of the Bahamas for the next two days, on a trajectory that would take it near the US coast.

Forecasts have prompted the governor of South Carolina, Nikki Haley, to warn a mass evacuation order will likely be issued later today unless there is a major change in the hurricane's path.

It would be the first major evacuation since Hurricane Floyd in 1999.

A vehicles sits next to a house, stranded in the flood waters, in Leogane, Haiti | Photo: PA Images

Florida governor Rick Scott has urged residents along the Atlantic Coast to prepare for a direct hit, possibly by Thursday night, and to stock up on three days' worth of food, water and medicine.

He said: "Don't take a chance. Leave before it's too late. We have to be prepared to be hit by a catastrophic hurricane."

President Barack Obama has postponed a Hillary Clinton campaign event in the Sunshine State later today.

The weather warnings have triggered a rush by Americans to stock up on food, gasoline and other emergency supplies.

Irish aid agency Concern has 128 staff based across Haiti and has stockpiled urgent supplies for communities in need as the storm hits.

Speaking from the capital Port-au-Prince, Concern Worldwide country director Nellie Kingston said there is “an incredible amount” of apprehension in a country where thousands still live in camps six years after a devastating earthquake that killed over 300,000 people and left over 1.5 million people homeless

“We have many life-saving items such as blankets, shelter equipment and aqua tablets because once the hurricane hits things like electricity and water supplies very quickly become compromised,” she said.

“People have just come through the most horrific earthquake in recent memory. They have been hit by cholera epidemics and now we are facing this.”

“The people of Haiti are amazingly resilient, but there is a feeling of: 'Why us?'”