The number of people killed by Hurricane Matthew in Haiti surged on Friday to 842, as help slowly trickled into marooned areas of the country’s southwestern peninsula pummeled by the powerful storm.

Officials told Reuters that they expect the number of deaths to increase, with isolated areas reporting higher numbers. Most deaths are believed to have occurred in the southwest region.

“Devastation is everywhere,” said Pilus Enor, mayor of Camp Perrin, a town near the port city of Les Cayes on the peninsula’s south shore.

“Every house has lost its roof. All the plantations have been destroyed. This is the first time we see something like this.”

Hurricane Matthew has pounded the Bahamas after slicing through Haiti and Cuba.

Trees and power lines were reportedly down in the Bahamas but no fatalities were reported.

Most of the deaths in Haiti were in towns and fishing villages around the southern coast, with many killed by falling trees, flying debris and swollen rivers.

The storm passed directly through the Tiburon peninsula, driving the sea inland and flattening homes with winds of up to 230km/h (145mph) and torrential rain on Monday and Tuesday.

The collapse of an important bridge on Tuesday had left the south-west largely cut off.

Non-governmental organisations said phone coverage and electricity were down and people were running out of food and water.

The BBC’s Tony Brown in south-western Haiti said he had seen people trying to cope with the mass destruction on their own, trying to rebuild from the rubble but without the help of the army or police.

Jean Joseph, a resident in one of the worst-hit towns described the scene as “complete devastation”.

“What’s going on right now is a lot of people are walking around, they have no home, a lot of them – they’re just walking around. I don’t know what they’re going to do,” he said.

The UN Office for the Co-ordination of Humanitarian Affairs, OCHA, reported that across the country, there were some 350,000 in need of assistance.

A spokesperson for the American Red Cross, Suzy DeFrancis, said the first priority was to get phone networks across the country back up and running, promising that “We will bring in technology to help do that.”

“… and then we are most worried about cholera, so we will be helping to distribute aqua tabs to purify the water,” she added.

The Red Cross has launched an emergency appeal for $6.9m “to provide medical, shelter, water and sanitation assistance to 50,000 people”, while the US is sending nine military helicopters to help deliver food and water to the hardest-hit areas.

Haiti is one of the world’s poorest, with many residents living in flimsy housing in flood-prone areas.

Meanwhile in the US, evacuation orders have been issued for areas covering some three million inhabitants.

The storm has so far stayed off the coast, lessening the potential impact to southern Florida, and it remains unclear when, where or if, the hurricane will make landfall.

Nevertheless, rain and high winds lashed the Miami area overnight.

Some 270,000 homes and businesses in Florida are already left without power.

Florida Governor, Rick Scott said: “Think about this: 11ft (3.3m) of possible storm surge. And on top of that, waves. So if you are close, you could have the storm surge and waves over your roof.”

One person is reported to have died so far in the US as a result of the storm.