The United Nations has warned that it could take days for the full impact of Hurricane Matthew in Haiti to emerge, as the death toll soars to more than 800, said a report.

The death toll has doubled, and may rise, as rescue teams gain access to southern areas cut off by the storm, according to www.bbc.com.

The World Food Programme's Carlos Veloso says some of the hard-hit towns can only be reached by air or sea.

Many of the deaths in Haiti were in the south-western coast, which suffered the full force of the hurricane this week.

Hurricane Matthew is currently battering the coastline of the US state of Florida but has been downgraded to a Category Two storm, with sustained wind speed dropping to 177km/h.

Category Five is the strongest on the five-step Saffir-Simpson scale of hurricane intensity.

Rescue efforts are under way to assess the destruction left in the wake of the most powerful Caribbean storm in a decade.

Haiti's Civil Protection Agency on Friday doubled the death toll from the hurricane from 400 to more than 800.

Three other towns in the south are reporting dozens of fatalities, according to Reuters news agency. The mayor of the village of Chantal told the news agency that 86 people had died and 20 more were missing.

Civil Protection Agency official Saint-Victor Jeune said his team had found another 82 bodies in the mountainous outskirts of Jeremie. But they were unable to register these with the Haitian authorities because of poor communications, he said speaking to Associated Press news agency.

The storm passed directly through the Tiburon peninsula - encompassing Haiti's entire southern coast - driving the sea inland and flattening homes with winds of up to 230km/h and torrential rain.

Government and UN officials estimate that some 350,000 people are in need of assistance.

Non-governmental organisations say communication with the areas worst affected has been hampered by lack of phone coverage and downed power lines.

The US is sending its USS Mesa Verde navy ship to assist with rescue efforts, as well as nine military helicopters to help deliver food and water to the hardest-hit areas.

The Red Cross has launched an emergency appeal for $6.9 million "to provide medical, shelter, water and sanitation assistance to 50,000 people".

This comes amid concerns about a surge in cholera cases, with the sanitation system in Haiti already overwhelmed.

Haiti - one of the world's poorest countries - has never fully recovered from the earthquake that killed thousands of people in 2010 and a cholera epidemic that followed.