Washington - A storm threatening the US coast was upgraded to an "extremely dangerous" Category 4 hurricane, the US National Hurricane Center (NHC) announced early Wednesday.

"This is a life-threatening event for portions of the north-eastern Gulf Coast," the NHC wrote on Twitter.

Hurricane Michael was expected to make landfall over the Florida panhandle on Wednesday, while bringing winds and heavy rain across portions of southern Georgia and south-east Alabama, it said.

"Its outer rainbands are beginning to reach the coast," it added.

Heavy rainfall could also produce "life-threatening" flash flooding in Florida, Georgia, North Carolina, South Carolina and Virginia, the weather service said.