Hurricane Matthew has twisted toward the Bahamas and Florida's east coast after killing at least 26 people and damaging a majority of homes in Haiti's south.
The disaster has prompted the hard-hit country to postpone a long-awaited presidential election.
The powerful Category 3 hurricane, the fiercest Caribbean storm in nearly a decade, whipped Cuba and Haiti with 230km/h winds and torrential rains, pummelling towns and destroying livestock, crops and homes.
In the United States, more than 1.5 million people were urged to evacuate the southeastern coast and Florida Governor Rick Scott warned residents to prepare for a possible direct hit that could be catastrophic.
Hundreds of thousands of people had been evacuated from the path of Matthew, which caused severe flooding and killed four people in the Dominican Republic, as well as at least 22 in Haiti. The two countries share the island of Hispaniola.
The storm carved a path of devastation through southwestern Haiti, dumping boats and debris on coastal roads hit by surging seas and flooding residential areas heavily.
Some 80 percent of homes were damaged in Haiti's Sud Department, which has a population of more than 700,000, a government official said in a meeting with UN officials. Some 11,000 people were in shelters in the province.
In the town of Jeremie, people were cooking and sleeping outside because most houses were either knocked down or severely damaged. Similar scenes were reported across the coastal towns of the south.
Haiti, the poorest country in the Americas, had been set to hold a repeatedly postponed presidential election on Sunday, but the country's electoral council delayed it again in the aftermath of Matthew. No new date has yet been set.
Storm could strengthen
Matthew was downgraded early on Wednesday from a Category 4 storm on the five-step Saffir-Simpson scale of intensity, the US National Hurricane Center said.
The storm had sustained winds of about 185km/h on Wednesday evening, but the NHC said it could strengthen as it approaches Florida.
The eye of the storm was about 435km south-southeast of Nassau in the Bahamas at 8pm EST (1pm NZT) and was expected to be very near the east coast of Florida by Thursday evening, the NHC said.
"Everyone in our state must prepare now for a direct hit," Florida Governor Scott told a news conference in Tallahassee. "If Matthew directly impacts Florida, the destruction could be catastrophic and you need to be prepared."
It was difficult to assess the full extent of Matthew's impact in Haiti because it knocked out communications in many of the worst-affected areas, including the main bridge that links much of the country to the southwest peninsula.
Haiti was a particular concern because it is prone to mudslides due to extensive deforestation, and tens of thousands of people are still living in tents and makeshift dwellings after the 2010 earthquake that killed more than 200,000 people.
A spokesman for UN Secretary-General Ban Ki-moon said the Haitian government estimated 350,000 people needed immediate assistance.
Chris Bessey, country representative at charity Catholic Relief Services, said on-the-ground reports from the small coastal town of Dame Marie indicated that much of the town had been destroyed.
The death toll in Haiti included at least eight people killed by falling trees and six others swept away by swollen rivers, authorities said.
Mourad Wahba, the UN secretary-general's deputy special representative for Haiti, said the storm had triggered "the largest humanitarian event" witnessed in Haiti since the 2010 quake.
The US government said it was ready to help the afflicted, and about 300 US Marines set off on the USS Mesa Verde to provide disaster relief in Haiti, the Marines said on Twitter.
Cuba's oldest town gutted
There were no immediate reports of deaths in Cuba but Matthew devastated the picture-postcard tourist town of Baracoa, Cuba's oldest colonial settlement, in the province of Guantanamo. It gutted many houses there, dumping hunks of cement, wooden beams, roof tiles and fallen electrical lines on the streets.
"This is unbelievable. The little we have we tried to look after, we know we don't have the means to get more," said Hector Toirac, 45, a carpenter in a nearby fishing village where residents rummaged for belongings in the rubble of collapsed homes.
The storm passed close to the US naval base and military prison in Guantanamo and was on track to plow through the central and northwestern Bahamas, the NHC forecasts showed.
Officials in the Bahamas urged residents to evacuate to higher ground and the Ministry for Grand Bahama said on Facebook that government offices in New Providence and Grand Bahama were closed until further notice.
Hurricane and tropical storm warnings have been extended along a large stretch of Florida's east coast ahead of Matthew's forecast arrival there on Thursday.
Tropical storm or hurricane conditions could affect parts of Georgia, South Carolina, and North Carolina later this week, even if the centre of Matthew remained offshore, the NHC said.