A severe storm system threatened tens of millions of people in Mississippi and the US Southeast on Monday, a day after tornadoes killed 17, ripped up homes and flipped cars over in nearby states.

A tornado swept through the northern city of Tupelo around 2.30 pm (1930 GMT), the National Weather Service said, adding that a crew was en route to survey the damage.

Parts of Alabama were also at high risk of severe storms, with a moderate risk affecting portions of Louisiana, Tennessee and Mississippi as the system traveled east, with numerous tornadoes expected.

In the hardest-hit parts of Arkansas, emergency crews intensified their search for survivors, as residents of the close-knit community of Vilonia surveyed the damage.

Vilonia police chief Brad McNew said the town of 4,000 had been rendered unrecognizable.

"It's houses completely down to the foundations," he told NBC television.

Through the night, rescuers used searchlights in blacked-out areas, sifting through mountains of rubble in the hopes of finding someone alive.

The Arkansas Department of Emergency Management said 14 people had been killed in the state -- an earlier figure of 15 was revised as one victim was counted twice -- while an official in Oklahoma state said there were at least two tornado victims there.

Local media reported another fatality in the state of Iowa.

McNew said more would have been killed if not for emergency sirens that warned people the twister was about to hit.

"The storm warnings went out fairly early," McNew said.

"I went to a tornado shelter myself with my family which was a couple miles away from where we were at. A lot of people in the community were there. And so, it did work," he said.

"If you see the destruction that is here, even though we've lost some lives, there are many lives that was saved because of the storm warnings."

Vilonia was struck three years ago by a tornado that took almost the same path, but Sunday's twister was "a lot worse," McNew said.

Twisters also devastated large sections of the town of Mayflower, population 2,300, just northwest of the Arkansas state capital, Little Rock.

- 'Your country will be there' -

The National Weather Service warned that some of the new group of tornadoes could be "intense," with "very large hail and damaging straight line winds" also likely.

Forecasters warned the twisters would continue to threaten the central and southern United States through Tuesday, and said powerful thunderstorms and severe flooding were also possible.

Speaking in the Philippine capital Manila during an Asian tour, US President Barack Obama offered condolences and promised federal government aid.

"I want everybody to know that your country will be there to help you recover and rebuild as long as it takes," he said.

The White House said Obama called Arkansas Governor Mike Beebe to offer federal assistance.

Sunday's tornado crushed large trucks like empty cans, homes were violently ripped in half, and entire residential blocks were reduced to rubble.

Some homes were uprooted from their foundation. In Iowa, the tornado also dumped heavy rain, snapped trees and lifted the roof off a medical center in the town of Oskaloosa.

Dozens of homes were also reported destroyed in nearby Kansas, although officials so far have reported no fatalities there.