Haiti began three days of mourning Sunday for hundreds killed in Hurricane Matthew as relief officials grappled with the unfolding devastation in the Caribbean country's hard-hit south.

Matthew was downgraded Sunday to a post-tropical cyclone after cutting a swath from Florida to North Carolina that left at least 17 dead.

As of 11 a.m. Eastern time, the storm was packing 75 mile per hour winds as it moved away from the North Carolina coastline, although it was still described as packing a punch.

Behind it lay widespread flooding, washed out roads, downed power lines and trees and other havoc. Still, the United States seems to have dodged a bullet: as recently as Thursday night the predictions of storm damage bordered on the apocalyptic.

Attention shifted 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 winds.

Civil defense officials have put the death toll at 336, although some officials said it topped 400.

As southern Haiti has a population of 1.3 million, with a poverty rate of 60 to 70 percent, "we are not far from having 1 million people who are in urgent need of humanitarian assistance," said Mourad Wahba, the UN humanitarian coordinator in Haiti.

Aerial footage from the hardest-hit towns showed a ruined landscape of metal shanties with roofs blown away, downed trees everywhere and mud from overflowing rivers covering the ground.

The hurricane destroyed at least 80 percent of crops in some areas, so people will head from the countryside into cities. And the slums of cities like Port-au-Prince and even towns in the south will only grow and worsen, said Wahba.

He warned, therefore, that aid must not target only cities and towns but also go to farmers to keep them working their land.

"We learned from 2010, but are we going to put those lessons into practice? I don't know," said Wahba.

Interim President Jocelerme Privert declared three days of national mourning for the dead.

Clothes and mattresses are set out to dry on the roofs of destroyed houses after Hurricane Matthew passes in Jeremie, Haiti, on Oct. 8. Credit: Carlos Garcia Rawlins/Reuters

Related: Hurricane Matthew's destructive path through Haiti (PHOTOS)

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.

On Saturday a plane chartered by the United States brought in the first part of more than 480 tons of aid that America has pledged.

France announced it was sending 60 troops, with 32 tonnes of humanitarian supplies and water purification equipment. Two French helicopters are already in Haiti to help with reconnaissance flights and the transport of aid material.

California-based charity International Relief Teams said it was donating $7 million in medical supplies with international organizations MAP International and Hope for Haiti.

Flooding

In the United States, coastal flooding from the storm surge posed the biggest threat to life and property.

Water levels should subside by this evening in North Carolina, and the storm should gradually weaken over the next day or so, 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.

Millions of Americans were subject to evacuation orders and curfews were slapped on cities as the lethal storm barreled north after storming through Haiti, the Dominican Republic, Jamaica, Cuba and the Bahamas.

President Barack Obama had declared federal states of emergency in Florida, Georgia, and North and South Carolina.

More than a million people were left without power, which was slowly being restored.

In Jacksonville Beach, Sam Vond, a 68-year-old retiree, calmly rode his bike along the shore Saturday.

"It wasn't that bad. I didn't go outside. We stayed in the house until we were told it was OK to get out. Luckily, no damage to my house, so I'm happy," he said.

Matthew damaged roofs at the Kennedy Space Center but spared Florida's heavily populated south-central coast a direct hit.

Cities including Savannah, Georgia, and Charleston ordered dusk-to-dawn curfews to keep people off the streets and guard against looting.