The devastation Hurricane Matthew caused in Haiti is slowly beginning to emerge, with at least 877 people killed and tens of thousands left homeless by the monster storm.

The United States was bracing for the impact of Matthew over the weekend but the storm weakened after battering the coast of Florida and its severity has since been downgraded from a Category 4 to a Category 2.

Matthew, carrying winds of 195 kilometres per hour, hit Florida on Friday night (Australian time) and triggered mass evacuations of millions of people along the coast from Florida through to Georgia and into South Carolina and North Carolina.

The storm killed at least four people in Florida and knocked out power to more than 1 million homes and businesses. Two people were killed by falling trees, while an elderly couple died of carbon monoxide poisoning from a generator while sheltering from the storm inside a garage.

The Emberton family walk through debris along highway A1A in Flagler Beach, Florida.

An official vehicle surveys the damage to Highway A1A.

Southern parts of Florida escaped the brunt of the storm, but authorities urged people further north not to become complacent, and US President Barack Obama urged people to heed local officials’ instructions.

“I just want to emphasize to everybody that this is still a really dangerous hurricane, that the potential for storm surge, loss of life and severe property damage exists,” Mr Obama said after a briefing with emergency management officials.

In Haiti, the poorest country in the Americas, the death toll surged to at least 877 people as information trickled in from remote areas previously cut off by the storm.

Tens of thousands in the impoverished Caribbean nation have also been left homeless by the monster storm, which smashed through Haiti’s western peninsula on Tuesday with 233 km/h winds and torrential rain.

More than 61,000 people were in shelters, officials said, after the storm pushed the sea into fragile coastal villages.

While highlighting the misery of underdevelopment in Haiti, which is still recovering from a devastating 2010 earthquake, the storm looked certain to rekindle the debate about global warming and the long-term threat posed by rising sea levels.

Jeremie in Haiti, was one of the cities that suffered the greatest destruction as a result of Hurricane Matthew.

At least three towns in the hills and coast of Haiti’s fertile western tip reported dozens of people killed, including the farming village of Chantal where the mayor said 86 people died, mostly when trees crushed houses. He said 20 others were missing.

“A tree fell on the house and flattened it. The entire house fell on us. I couldn’t get out,” said driver Jean-Pierre Jean-Donald, 27, who had been married for only a year.

“People came to lift the rubble, and then we saw my wife who had died in the same spot,” he said, his young daughter by his side, crying.

With mobile phone networks down and roads flooded, aid has been slow to reach hard-hit areas. Food was scarce and at least seven people died of cholera, likely because of flood water mixing with sewage.

The Mesa Verde, a US Navy amphibious transport dock ship, was heading for Haiti to support relief efforts. The ship has heavy-lift helicopters, bulldozers, fresh water delivery vehicles and two surgical operating rooms.

-with AAP