Matthew made landfall in Haiti on Tuesday as a Category 4 hurricane, tearing through the small Caribbean nation with 125 mph (200 kph) winds and heavy rains that flooded villages, razed crops, swept away cattle and cut off the parts of the island.
It's the strongest storm to hit Haiti in a half century, in a country still reeling from the 2010 earthquake.
Haiti's interim President Jocelerme Privert has declared three days of national morning for the hundreds of victims.
The official toll is at least 336, according to a spokesman for Haiti's Civil Protection Service, Joseph Edgard Celestin.
"We do not know the exact number, we cannot find all the people," Haitian Sen. Herve Fourcand said Saturday.
Other media outlets report much higher death counts. A count by Reuters, based on information from local civil protection officials, puts the death toll well over 800.
According to the United Nations, more than a million people have been affected by the disaster, a tenth of Haiti's population.
Aid workers are scrambling to gain access to some of the country's hardest-hit areas.
UNICEF says an estimated 750,000 people need humanitarian assistance.
Aerial View of Damage
Haiti's southwestern peninsula is one of the most isolated parts of the country and hardest hit by the hurricane. An aerial view gives a sense of of the scale of the damage there.
Six years ago, the region was left largely untouched by the earthquake that shattered the Haitian capital.
This time, the residents were not so lucky.
People living in Port Salut cleaned much of the debris off the roads after the storm, but at night they sleep outside, in the dark. Mangled electrical wires dot the landscape.
Raoul Roa, a Port Salut resident, said he does not know when electricity would be back up again. He said his house was broken down and everything he had was gone.
People say they have had to wait days for emergency medical care.
UN officials said the hurricane is the country's worst humanitarian crisis since the devastating 2010 earthquake that killed more than 200,000.
The focus now is getting aid to the people who were affected, especially food and water.
There are warnings this could worsen the nation's cholera epidemic, which killed at least 10,000 people after the 2010 earthquake.
"Cholera is the biggest problem right now," Fourcand said. "We need clean water, the water here is so dirty."
An estimated 500,000 children live in the areas worst hit by Hurricane Matthew, UNICEF said. UNICEF representative in Haiti Marc Vincent said they are "still far from having a full picture of the extent of the damage," as they "are hoping for the best, but bracing for the worst."
The U.S. State Department warned citizen travelers of "serious problems concerning emergency response/medical care infrastructure and crime in Haiti," in a statement released Friday.
"Food insecurity is going to be a serious, serious problem" warned Joseph Alliance of the aid group Action Aid. "In terms of economic impact, agriculture is wiped away, 95% destroyed," he added.
"The livelihoods of the people right now doesn't exist."