Hurricane Matthew killed more than 800 people and left tens of thousands homeless in Haiti earlier this week before it skirted Florida's Atlantic coast on Friday and plowed northward over waters just off Georgia.
The number of deaths in Haiti, the poorest country in the Americas, surged to at least 842 on Friday as information trickled in from remote areas previously cut off by the storm, according to a Reuters tally of death tolls given by officials.
Matthew triggered mass evacuations along the U.S. coast from Florida through Georgia and into South Carolina and North Carolina.
U.S. President Barack Obama urged people not to be complacent and to heed safety instructions.
"The potential for storm surge, loss of life and severe property damage exists," Obama told reporters, after a briefing with emergency management officials about the fiercest cyclone to affect the United States since Superstorm Sandy four years ago.
Matthew smashed through Haiti's western peninsula on Tuesday with 145 mile-per-hour (233 km-per-hour) winds and torrential rain. Some 61,500 people were in shelters, officials said, after the storm pushed the sea into fragile coastal villages, some of which were only now being contacted.
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 to low-lying cities and towns by rising sea levels.
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," Jean-Donald said, his young daughter by his side, crying "Mommy."
With cellphone networks down and roads flooded, aid has been slow to reach hard-hit areas in Haiti. Food was scarce, and at least seven people died of cholera, likely because of flood water mixing with sewage.
The Mesa Verde, a U.S. 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.