Florida hunkered down while the hurricane unleashed torrential rain, potentially devastating surges and 195 kmph winds, leaving one person dead and 600,000 homes without power.
Fears of a potentially catastrophic impact - which triggered mass evacuations up and down the coast - did not materialize as Matthew brushed Florida's coast overnight, lashing Cape Canaveral, home to the Kennedy Space Center.
But Florida Governor Rick Scott warned at a morning news conference, "the worst effects are still likely to come," as the storm churns up the coastline as far as North Carolina.
"I emphasise this is still a really dangerous hurricane," warned President Barack Obama, who declared a federal emergency for Florida and South Carolina as the storm barreled in from the Caribbean.
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"The bigger concern at this point is not just hurricane force winds, but storm surge," Obama said.
As Americans battened down, the full scope of the disaster in impoverished Haiti was becoming clearer.
Herve Fourcand, a senator for the Sud department which felt the full force of Matthew's impact, said he had recorded 400 deaths with several localities still inaccessible.
In Jeremie, a town of 30,000 people, 80 per cent of buildings were knocked down, said non-governmental organisation Care.
Further south, the town of Les Cayes, Haiti's third largest, was battered.
"I thought I was going to die. I looked death in the face," said 36-year-old Yolette Cazenor, standing in front of a house smashed in two by a fallen coconut palm.
"Everyone is a victim here, houses have been washed away, we lost all the roofing. I lost everything," added Dominique Osny.
The storm was only the latest natural disaster to ravage the Caribbean nation, which in January 2010 was hit by a devastating earthquake that demolished much of the capital and left more than 250,000 dead.
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