Haitian officials are struggling with mounting death tolls, and the devastation is massive. Health officials have warned of a surge in cholera cases due to widespread flooding.
Local officials in Haiti estimate that at least 339 people had been killed by the storm, with thousands more displaced.
A large number of the dead, at least 50, were believed to have come from one town in the hard-hit south of the country.
Authorities have warned that the number of dead is expected to rise further. At least seven others were killed elsewhere in the Caribbean.
Officials said many of those killed in Haiti were hit by falling trees, flying debris or caught up in swollen rivers.
Rescue workers were still trying to assess the damage, as they struggled to reach isolated towns because roads were impassable.
"I've never seen anything like this," said Louis Paul Raphael, a central government representative in the Roche-a-Bateau.
Massive structural damage
"The storm ripped off roofs, teared down electricity posts, a lot of people were looking for shelter in churches and schools," Doris Wasmeier, a German visitor in Port au Prince, told the DPA news agency.
"We don't know how the situation in the south is. Communication has broken down and the most important bridge came down. The region is totally cut off," said Wasmeier, who was an aid worker during the 2010 earthquake in Haiti that killed tens of thousands.
Sunday's presidential election was postponed due to the devastation. A new date for the elections has not yet been set.
Surge in cholera cases
UN health officials have warned of an upsurge in cholera cases due to widespread flooding and damaged sanitation systems.
Haiti has struggled to overcome a cholera outbreak ever since it was introduced into the country's biggest river from a UN base. International peacekeeping forces were deployed in the country after it was hit by a devastating earthquake in January 2010, which demolished the capital and killed more than 250,000 people.
The disease has killed around 10,000 people and infected more than 800,000 since.
Bahamas and US next in storm's path
After passing Haiti, Matthew barreled across part of Cuba before hitting the Bahamas ahead of its progress to the US eastern seaboard.
Some 3 million people were ordered to leave their homes on Thursday in anticipation of Hurricane Matthew's expected direct hit on Florida later in the evening.
US President Barack Obama issued a state of emergency for Florida, as forecasters said winds of up to 220 kilometers per hour (140 mph) were likely to slam into the US' southeast coast, although the speed appeared to be dropping slightly. Nevertheless, the storm still has the potential to cause billions of dollars of damage to property and infrastructure.
"This will kill you," Florida Governor Rich Scott warned those residents who were unsure about leaving the area, telling them that "time is running out." In addition to regular radio and TV alerts, residents have been advised to download the "Florida Evacuates" app.
mm,rc,dm /bw,kl (AFP, AP, dpa, Reuters)