At least 276 people have been killed so far in Haiti, the Dominican Republic and St. Vincent and the Grenadines, officials said, with the death toll expected to rise.
As the death toll rises and crucial infrastructure crumbles, hundreds of thousands have been displaced.
Mourad Wahba, the UN secretary-general's deputy special representative for Haiti, described Matthew as the "largest humanitarian event" since the earthquake.
Four people died in the Dominican Republic, the country's government said. Authorities there did not provide details about how they died.
In St. Vincent and the Grenadines, a teenage boy died in a landslide as he was cleaning a drain behind his house, the National Emergency Management Office said. He died Wednesday after storms from Matthew passed.
Haiti: 'A total disaster'
Hurricane Matthew made landfall in Haiti on Tuesday but the details of its devastating impact are slowly coming to light.
At least 1,580 homes have been flooded in the country, and about 3,215 families have been affected by the severe storm, the country's Civil Protection Agency said.
More than 300,000 people are in shelters across the country, the United Nations said.
Aid agencies are finding that the impact has been far greater than expected.
Ariane Quentier, spokeswoman for the UN Stabilization Mission in Haiti, said: "What we are realizing, as we are going further ... is that there's been a lot of devastation and a lot of damage and probably a lot more than what we had initially assessed."
This means the initial aid already sent to the country's worst-affecetd areas in the west and south have fallen short of the need. Evacuation centers are overwhelmed and communities and workers have been forces to set up more and more makeshift shelters with what resources they have.
Haitian pastor Louis St. Germain said the storm sheared a wall off his house and tore roofs off many buildings in the area.
"The river has overflowed all around us," St. Germain said. "It's terrible ... a total disaster."
Southern Haiti was hit especially hard, where winds of 125 mph (200 kph) destroyed homes, flooded villages and cut off the island from the rest of the country.
National Route 2, which connects Port-au-Prince with Haiti's southern peninsula, broke apart when the bridge collapsed, the country's civil protection agency said.
In the wake of the storm, the Electoral Commission postponed the country's presidential election, which had been scheduled for Sunday. A new date has not been set.
Tim Callaghan, assistance response team leader for the US Agency for International Development in Haiti, told CNN that much of the damage in Haiti's hardest hit areas -- the southwestern cities of Les Cayes and Jeremie -- appeared to be structural, and not the result of heavy rainfall.
"The urgent need we're focusing on right now is food, safe drinking water and ... things like plastic sheeting, hygiene kits and so forth," he said.
"We're in the most critical phase to support people."
Dominique Fevry-Gilliand, a spokeswoman for Oxfam Canada, said up to 80% of homes in the most affected areas were destroyed. There was also extensive damage to crops along swaths of southern Haiti.
"Most likely, in the next couple of weeks and months, one of the things they will have to deal with in that region is hunger," she said.
Haiti was only just beginning the "development" phase to rebuild infrastructure following the 2010 earthquake that destroyed much of the country, said Ines Brill, the International Federation of Red Cross and Red Crescent Societies representative for Haiti.
"This is an emergency after an emergency, which makes things more complex than a typical disaster," she told CNN.
Bahamas: Damage still unknown
The powerful hurricane passed over the Bahamas capital, Nassau, on Thursday afternoon.
While no injuries had been reported early on Friday morning, houses had suffered flood and wind damage and a number of roads were impassable due to collapsed trees and poles.
The country's national emergency agency was telling people to stay indoors until an assessment could be made of the damage.
"One of the main roadways in front of Sandals (resort) has been blocked off by debris and fallen trees," Nassau resident Denzil Sirra told CNN, adding his house had not been damaged.
"A lot of debris and fallen trees and damaged shrubs. No electricity right now. Still have running water."
Officials said the hurricane caused flooding in southern and eastern coastal communities and structural damage to a number of resorts in Nassau.
CNN meteorologists said storm surges in the Bahamas reached as high as 15 feet, along with intense rains and damaging winds.
Cuba: Dozens of houses washed away
Hundreds of people had the roofs blown off their homes as Hurricane Matthew swept across the country, Cuban state media reported, while photos from the seaside town of Baracoa showed devastation in the severe storm's wake.
More than 30 houses were washed away by the hurricane in the northeastern Cuban town, the site where Christopher Columbus first landed in the Americas, a resident in the town said.
No fatalities were reported as of Thursday evening, local time, as the seafront area of the town had been evacuated ahead of Matthew's arrival.
The United Nations has offered support to Cuba in the aftermath of Hurricane Matthew, in coordination with the Cuban authorities, state media
reported on Thursday.
US braces for impact
Officials in the United States have taken steps to prepare for the storm's arrival
. Governors in four Southern states have declared states of emergency.
"I cannot emphasize enough that everyone in our state must prepare now for a direct hit," Florida Gov. Rick Scott said. "Having a plan in place could mean the difference between life and death."