Hurricane Matthew has hit Cuba and Haiti, flattening houses, flooding streets, and wreaking havoc with winds reaching 140mph.
At least four people were killed in the Dominican Republic by collapsing walls and mudslides, as well as two in Haiti.
"Haiti is facing the largest humanitarian event witnessed since the earthquake six years ago," said Mourad Wahba, the UN Secretary-General's Deputy Special Representative for Haiti.
Southern Haiti has been cut off from the rest of the country after a bridge collapse, authorities said.
Route Nationale 2, leading to and from La Digue Bridge, is the only road linking Port-au-Prince with the southern peninsular, which has been hit hardest by the deadly storm.
Civil protection spokesman Edgar Celestin told AFP: "An emergency meeting is taking place to restore access, but it's going to be difficult to find an alternative route,"
Matthew is expected to remain powerful as it moves across Cuba, the Bahamas, and even parts of the east coast of the United States.
Matthew signals the beginning of another humanitarian disaster in the poorest nation in the western hemisphere.
There are still 55,000 people living in makeshift tents and shelters as a result of the earthquake which struck in 2010.