Rescuers on Monday hunted for survivors of powerful tornadoes that killed at least 17 people when they ripped through small towns in the southern and central United States, with forecasters warning more were on the way.
Violent thunder and hail storms were also set to move through a large swath of territory from Texas to Tennessee, forecasters said, a day after tornadoes ripped up homes, flipped cars and uprooted trees.
In one of the towns hardest-hit -- Vilonia, in Arkansas -- emergency personnel intensified their search for survivors, as the arrival of daylight allowed harrowed residents of the close-knit community their first look at the destruction.
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 than it was three years ago," McNew said.
Twisters also devastated large sections of the town of Mayflower, population 2,300, just northwest of the Arkansas state capital Little Rock.
The National Weather Service warned of the threat of "a severe weather outbreak" beginning late Monday affecting several states, including the lower Mississippi Valley, Tennessee Valley and Central Gulf Coast states.
"Numerous tornadoes are expected, some of which could be intense," the NWS said.
"Very large hail and damaging straight line winds are 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, US President Barack Obama, who is on a tour of Asian nations, 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.
Television news footage showed large trucks crushed like empty cans in Arkansas, homes violently ripped in half, and entire residential blocks 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.
"There have been numerous homes and buildings damaged and some destroyed," Keli Cain of the Oklahoma Emergency Management Agency told AFP.
Dozens of homes were also reported destroyed in nearby Kansas, although officials so far have reported no fatalities there.