The storm system covering a large swath of the south-east was moving in a north-easterly direction, according to the National Weather Service.

The service had issued a tornado watch until 1500 GMT for the eastern coastline from northern Florida through Virginia and eastern Maryland. Parts of those states were also under a severe thunderstorm warning.

In the southern state of Alabama, eight teenagers were killed when a tornado struck a high school in the town of Enterprise, CNN reported, citing the town's mayor.

US television showed footage from Enterprise of roofless homes with blown-out windows, uprooted and shredded trees, dangling power lines and cars overturned and crushed.

High school student Brooke Shroades survived the tornado by hunkering down in a cubbyhole.

"I felt like I was on a rollercoaster. It was the scariest thing ever," she told the Enteprise Ledger newspaper.

"When I heard the train sound, I started screaming," she said.

Her father, Mike Shroades, who had hoped to pick up his daughter before the storm hit, took shelter in a school hallway with other parents, teachers and students.

"You could feel your body moving from the wind and suction," he told the Enterprise Ledger.

Alabama authorities revised downward an earlier toll of up to 19 dead, US media reported.

"It is complete devastation," Sheriff Russell Thomas told WSFA television as he assisted in rescue efforts in Enterprise. "It is very tragic."

There is a "possibility" that more bodies will be found as the sun rises, John Tallas, the county's top emergency official, told CNN.

The authorities also said the storm killed two other people in other parts of the state.

A higher, incorrect toll for Alabama was earlier repeated by the White House. Alabama state authorities blamed the problem on miscommunication.

In the pre-dawn hours of Friday a tornado spawned by the same storm delivered a direct hit on the Murray Sumter Regional Hospital in the town of Americus, Georgia, destroying the ambulance fleet and forcing 55 patients to evacuate, CNN reported.

Two people not linked to the hospital were killed in Americus, according to CNN.

At least seven other people were killed across southern Georgia, and one girl was reported dead after an apparent tornado touchdown earlier in Missouri, US television reported.

News of the children killed in Alabama came Thursday as President George W Bush -- widely criticised for a slow response to Hurricane Katrina in 2005 -- visited New Orleans to tell the storm victims he had not forgotten about them.

"The administration stands ready to help," Bush spokesman Scott Stanzel.

Federal authorities announced they were sending emergency aid to storm-stricken Alabama and Missouri.

Stanzel added that Bush was "deeply saddened" by news of the teens' deaths.

The single biggest tornado strike was to Enterprise, a town of 22,000 that Alabama Governor Bob Riley said suffered "major and widespread damage." Riley announced he was deploying about 100 National Guard soldiers to assist in recovery efforts.

Local hospitals asked for blood donations and utility Alabama Power said about 15,000 homes across the state were without electricity.

In Enterprise, school official Bob Ferris said the extent of the tragedy was not immediately clear. "We need your prayers," he said at a news conference.

Dozens of people were taken to hospitals in Enterprise and other parts of the southern state, where numerous homes were reported destroyed.

The National Oceanographic and Atmospheric Administration (NOAA) said that 19 tornadoes touched down in Alabama, Missouri and Georgia on Thursday.

"Tornadoes, large hail and destructive thunderstorm winds will remain a threat in parts of the Southeast and possibly even the mid-Atlantic" through early Friday, NOAA said in a statement.