Several states along the east coast of the US are preparing for Hurricane Florence, which has been strengthening as it crosses the Atlantic.

The US National Hurricane Centre (NHC) said the storm is "expected to become a major hurricane very soon".

A major hurricane means the storm has wind speeds greater than 170km/h, with Florence expected to strengthen to that level today.

The NHC adds that there is an "increasing risk of life-threatening impacts" from the storm, including flooding from "exceptionally heavy" rainfall and hurricane-force winds.

Here are the 5 AM EDT Key Messages for Hurricane #Florence. Latest information available at https://t.co/tW4KeGdBFb pic.twitter.com/DpgFeN7OIZ

— National Hurricane Center (@NHC_Atlantic) September 10, 2018

While the exact path the hurricane will take remains unclear, it could hit the US coast on Thursday or Friday.

States of emergency have already been declared in North Carolina, South Carolina and Virginia ahead of possible landfall.

"Everyone in North Carolina needs to keep a close eye on Florence and take steps now to get ready for impacts later this week." - Gov. Cooper.

— Governor Roy Cooper (@NC_Governor) September 9, 2018

South Carolina's Governor Henry McMaster said people "must not hesitate to prepare for the possibility of a hurricane impacting our coast".

"This storm is too powerful and its path is too uncertain to take any chances," he warned.

People in vulnerable areas are being urged to prepare for potential evacuation now, in case they are ordered to leave their homes later in the week.

Virginia Governor Ralph Northam, meanwhile, warned residents to prepare for hurricane season, saying: "I'm sure like me you remember Puerto Rico, Florida, Texas and the Virgin Islands and the devastation they faced after last year's hurricanes. The images are hard to forget."

He also declared a state of emergency so officials "can begin to prepare state assets" ahead of any potential impact from Hurricane Florence.