Emergency teams in Haiti struggled to reach hurricane-ravaged areas cut off by washed-out bridges and mudslides after Hurricane Matthew roared over the nation's western tip and began a devastating island-hopping path that arcs toward the US coast.

The full extent of Matthew's blow to Haiti remained unclear, with communications almost fully severed to some regions in the Western Hemisphere's poorest nation - where tens of thousands of people still live in tents after an earthquake six years ago killed 200,000 people.

At least 11 deaths - including at least four in the neighbouring Dominican Republic - have been blamed on the hurricane, which packed winds of up 230km/h when it swept over Haiti yesterday and then moved toward Cuba and the Bahamas. A slightly weakened Matthew was expected to make landfall in Florida tomorrow.

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

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John Hasse, national director of the aid group World Vision in Haiti, estimated "thousands of houses destroyed or damaged" in the areas hardest hit. The Haitian Government said at least 350,000 people were in immediate need of help around the country, the United Nations noted. At the same time, fears rose of possible outbreaks of cholera or other diseases.

Samuel Darguin, a worker with the aid group Haitian American Caucus-Haiti, said relief convoys were prevented from reaching many areas because of bridges swept away by flood waters.

Local teams in the isolated regions were in "search-and-rescue mode," he said, with reports of people missing. That suggested the death toll could rise. The Reuters news agency, citing officials in various countries, placed the tally at 17 dead along Matthew's path through the Caribbean. The figure could not be independently verified.

International aid was underway, with a focus on shelter and clean water. The US Navy has sent three ships, including an aircraft carrier and a hospital ship, to Haiti. About 300 Marines were aboard the USS Mesa Verde, an amphibious transport vessel.

The United States announced US$1.5 million in aid available to the Caribbean countries in Matthew's wake. Disaster response teams were pre-positioned with some supplies, and were assessing the damage and urgent relief needs, said Kenneth Merten, the deputy assistant secretary for the Western Hemisphere and special coordinator on Haiti.

Beyond food, the aid includes blankets, shelter and kits for hygiene and water purification to help prevent the spread of cholera, said David Harden, an official with USAID.

Venezuela, battered by a major economic crisis, mobilised an airlift of humanitarian supplies, and the British-based group Oxfam concentrated on sanitation and clean water efforts.

Many people live in rudimentary shacks crowding the hillsides that are especially vulnerable to the high winds and lashing rains of a hurricane - as well as the landslides that often occur afterward in rain-loosened soil.

In Port-au-Prince, water remained knee-deep in some low-lying districts. Boats and other flotsam were wedged amid stores and homes.

The UN deputy special representative for Haiti, Mourad Wahba, called Matthew "the largest humanitarian event" in Haiti since the earthquake and said that "much of the population" has already been displaced.

Jean-Michel Vigreux, director in Haiti for the aid group Care, said estimates on damage have reached as high as US$1 billion even before the full extent of the devastation is tallied.

"It is very scary," he said in a statement.

A woman and a child walk in a waterlogged street as they head to a shelter under the pouring rain caused by Hurricane Matthew, in Leogane, Haiti. Photo / AP
A woman and a child walk in a waterlogged street as they head to a shelter under the pouring rain caused by Hurricane Matthew, in Leogane, Haiti. Photo / AP

Matthew's eye was about 110km south of Long Island in the Bahamas, with maximum sustained winds of 195km/h, the National Hurricane Centre reported. The storm - now downgraded to a Category 3 hurricane - was heading northwest at 20 km/h on a path that would take it through the Bahamas before spinning towards Florida.

Officials in the Bahamas urged residents to move to high ground amid the dozens of low-lying islands in the archipelago. In Cuba, the damage appeared limited as authorities spent days preparing, including moving people off threatened coastal areas.

At the Guantanamo Bay base on Cuba's eastern tip, about 700 family members of US personnel and others were evacuated before Matthew hit.

Images posted on social media showed raging, muddy floodwaters nearly overflowing river banks and driving wind and rain that bent palm trees nearly in half. The winds sheared roofs from ramshackle houses, uprooted trees, washed out bridges and clogged rivers and roads with debris.

In one dramatic video, a man shouted in English, "Pray for us," as Matthew lashed the small port of Les Cayes on Haiti's southwestern coast.

Chris Bessey, a Catholic Relief Services representative based in Port-au-Prince, said the storm blew part of the roof off the charity's two-storey building in Les Cayes and knocked out its generator. The main road to southern Haiti was cut because a bridge was washed out at Petit-Goave, he said from Port-au-Prince.

"Yet right up until the storm hit, we still heard many people saying, 'We're waiting on God' and not making preparations," said Hasse, the World Vision official. He said the charity has warehouses stocked with blankets, tarps, water cans and hygiene kits to assist nearly 100,000 people, but "we expect it to go well over a million to even millions of people affected by this."

Relief officials said many people rejected pleas to evacuate, fearing that they would lose their belongings to looters. For some, who changed their minds as the storm unleashed its fury, it was now too late to help them, the officials said.

In the vast Cité Soleil slum in Port-au-Prince, 130 children were evacuated from an orphanage ahead of winds and rain that authorities feared could ravage the densely populated area of tin shanties, open sewers and canals.

A long-delayed presidential election is scheduled for Monday, and the Government had no immediate plans to postpone it.