The devastating hurricane wiped some towns and villages off the map, the UN chief said, after pledging $120m in aid.
North Carolina is still reeling from the effects of Hurricane Matthew. At least 14 people have reportedly been killed in the worst flooding to have hit the southeastern state in the US.
Flooding has also affected other states in the region, including Florida, Georgia and Virginia.
Although Matthew has disintegrated, dropping the last of its rain over the Canadian provinces of Newfoundland and Nova Scotia, river levels in North Carolina remain at near record levels.
It is known that 355mm of rain fell in Fayetteville on Saturday alone. That is more than double the previous 24-hour record (173mm) set during the passage of Hurricane Floyd in September 1999.
Record river levels were recorded on the Black River near Tomahawk; Cape Fear River at William O Huske Lock; Little River at Manchester; Lumber River at Lumberton and the Neuse River at Smithfield.
Although the levels of these rivers are generally falling, they will remain above the major flood stage for several days.
Financially, Matthew is likely to be the most expensive cyclone of the 21st century, so far. In the US alone, insured losses, approximately half of the total losses, are estimated at $6bn.