Hurricane Matthew has killed more than 800 people and left tens of thousands homeless in Haiti, before it cut power and forced people to evacuate US state Florida.
The storm smashed through the tip of Haiti's western peninsula on Tuesday with 233km/h winds and torrential rain.
The number of fatalities 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 tally of death tolls given by officials.
About 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.
Relief agencies say in the city of Jeremie, 80 percent of the buildings had been levelled.
In Sud province, 30,000 homes were destroyed.
At least 175 people died in villages clustered among the hills and coast of Haiti's fertile western tip.
At least three towns reported dozens of fatalities, including the farming village of Chantal, where the mayor said 86 people perished, 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".
US sends aid, workers
The United States said it had sent teams of military and US Agency for International Development personnel to help in the country.
USAID sent teams to Haiti, Jamaica and the Bahamas to work with local authorities to coordinate disaster relief, White House spokesman Eric Schultz told reporters aboard Air Force One.
The Defense Department had about 150 people in Haiti now, and "weather permitting" that number will grow to a couple of hundred over the weekend, Schultz said.
The teams were distributing food and water, helping with transportation, and setting up first responder capabilities, he said.
The US military said on Friday the USS Mesa Verde, an amphibious transport dock ship, was heading toward Haiti to support relief efforts and would start helicopter flights to support USAID efforts on the island.
The ship can produce fresh water and has water delivery vehicles aboard.
Storm weakens over Florida, but remains dangerous
The US state of Florida, where winds were about 195km/h, recorded its first death in the storm, which also downed power lines leaving more than 400,000 people without electricity.
The hurricane weakened to a category 3 storm, but was still strong enough to bring down power lines and signs, and send unsecured objects flying.
Officials said the worst for the US was yet to come, and the city of Jacksonville was expected to bear the brunt.
Giving an update on the storm warnings, Mr Obama reminded people to keep following official advice, and to spare a thought for Haiti.
"I would ask all Americans to go to the American Red Cross and other philanthropic agencies to do what we need to do to help people in need," he said.
A state of emergency is in place in several states as Hurricane Matthew moves up the southeastern coast of the US.
Craig Fugate, director of the Federal Emergency Management Agency, said he was concerned that relatively light damage in the south of Florida so far could give people farther up the coast a false sense of security.
He said there was still a danger of storm surge, particularly in northern Florida and southern Georgia.
"They've never seen this kind of damage potential since the late 1800s," Mr Fugate said.
The mayor of St Augustine, in Florida's northeast, says the most serious problem now is the people who have ignored the order to evacuate.
Nancy Shaver says the authorities are now encouraging those people to stay where they are.
- Reuters / BBC