Matthew, meanwhile, lost its hurricane status, subsiding to a "post-tropical cyclone" after cutting a swath from Florida to South Carolina that left several dead.
At 1200 GMT, the storm was still packing winds that gusted to hurricane strength as it moved away from the US coastline.
But attention was shifting back to Haiti, the Americas' poorest country and one shattered by a 2010 earthquake and ravaged by a cholera epidemic.
Matthew crashed ashore on Haiti's southern coast on Tuesday as a monster Category 4 storm, packing 145 mile (230 kilometer) winds.
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Civil defense officials put the death toll at 336, although some officials said it topped 400.
Interim President Jocelerme Privert declared three days of national mourning for the dead.
As the death toll climbed, pledges of aid flooded in, with the United States announcing it was sending a Navy ship, the USS Mesa Verde, whose 300 Marines will add to the 250 personnel and nine helicopters already ordered to deploy to Haiti.
California-based charity International Relief Teams said it was donating $7 million in medical supplies with international organizations MAP International and Hope for Haiti.
In the United States, coastal flooding from the storm surge posed the biggest threat to life and property.
"The combination of a dangerous storm surge, the tide, and large and destructive waves will cause normally dry areas near the coast to be flooded by rising waters moving inland from the shoreline," the NHC said.
Matthew made landfall southeast of McClellanville, South Carolina, on Saturday as a weakened Category 1 storm, but it triggered serious inland flooding.
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