Hurricane Sandy has left 21 people dead across the Caribbean after battering the Bahamas, Jamaica, Haiti and Cuba with torrential rain and heavy winds.

Sandy was downgraded to a category 1 storm as it tore through sparsely populated low-lying islands in the Bahamas, knocking out power and blowing roofs off homes.

The Cuban government says the storm killed 11 people as trees fell and buildings collapsed in the Santiago de Cuba and Guantanamo provinces.

The storm killed nine people in Haiti, even though the country avoided a direct hit.

In Jamaica, one person was killed by falling rocks when the storm hit.

Winds and rains generated by Sandy were also being felt in southern Florida in the United States.

The US National Hurricane Centre says Sandy was packing winds of up to 140 kph when it was about 90 kilometres off Great Abaco Island in the Bahamas.

Sandy is forecast to remain a category 1 hurricane as it continues to move over the Bahamas, sending swirling rains and winds across the groups of islands.

Government officials in the Bahamas warned residents to stay indoors until the storm passed. There were no immediate reports of casualties or serious injuries.

Stella Maris Resort manager Joel Friese says the storm damaged buildings and downed trees on Long Island, in the south-east corner of the Bahamas.

The Bahamas Electricity Corporation, which supplies power to the island chain, says Sandy has caused a number of power outages.

Potent storm

The storm is expected to soon move past the Bahamas and is forecast to come ashore in the United States north-east early next week.

Forecasters say the late-season storm is expected to combine with a polar air mass, potentially producing a highly unusual and potent storm that could wreak havoc along the US east coast.

Weather trackers say the hardest-hit areas could span from the coastal Carolinas up to Maine, with New York City and the Boston area also potentially at risk.

Hurricane expert Jeff Masters says a landfall along the mid-Atlantic coast could trigger a "billion-dollar disaster".

"There are many questions surrounding this hurricane and its forecast, but I find it important to convey that Sandy's impacts will be widespread, no matter the location of landfall," he wrote in a blog.

The storm could hit an area of New England where Hurricane Irene caused severe damage last year.

Sandy is forecast to be weaker, but will also be moving slower, meaning it will likely bring more rain increasing its potential for damage.

The storm comes amid final preparations for the US presidential election, to be held on November 6.

Reuters

