More than 1 million people in the southeast US faced mandatory evacuation orders as hurricane Florence neared Category 5 strength on Tuesday, authorities said.

Florence was upgraded twice on Monday, from Category 3 to 4, prompting evacuation orders in the coastal areas of North Carolina, South Carolina, and Virginia. reports CNN.

Weather officials have warned not to underestimate the threat the storm poses.

The size of the hurricane-force winds doubled over 12 hours from 30 to 60 mph, the National Hurricane Centre (NHC) said Monday afternoon.

The wind field is expected to keep growing, which will increase the storm surge and inland wind threats, the service said in a statement.

"None of the guidelines suggest that Florence has peaked in intensity, and this is supported by a continuation of a low-shear environment and even warmer waters over the next 36 hours. Thus, the intensity forecast is raised from the previous one, bringing Florence close to Category 5 strength tomorrow," the NHC said.

Hurricane Florence
Customers line up to buy propane at Socastee Hardware store, ahead of the arrival of Hurricane Florence in Myrtle Beach, South Carolina, U.S. September 10, 2018.REUTERS

In North Carolina, six more counties were placed under mandatory evacuation orders on Monday night, hours after the first order in Hatteras Island.

In South Carolina, mandatory evacuation orders take effect on Tuesday at noon in eight counties along the state's 187-mile coastline, Governor Henry McMaster said.

In Virginia, mandatory evacuations will begin on Tuesday morning for about 245,000 residents in a portion of Hampton Roads and the Eastern Shore area, Governor Ralph Northam said.

Florence became a hurricane on Sunday with sustained winds of at least 74 mph, CNN reported.

By Monday morning, the NHC classified Florence as a "major" Category 3 hurricane, with sustained winds of at least 111 mph.

It was picking up speed over the Atlantic's warm waters, causing concerns about landfall and flooding from heavy rains afterward, possibly late Thursday or Friday.