US President Joe Biden said the government would do everything possible to help the tornado-ravaged central areas of the country.
Mr Biden has signed a federal emergency declaration freeing up funds for Kentucky, the worst-hit state.
He said the storms were among the largest in US history.
More than 70 people died in Kentucky in Friday night’s storms, including dozen in a candle factory, and the death toll is expected to rise to over 100.
Emergency teams are searching the rubble of the factory in the town of Mayfield, which was hit directly.
More than 40 people were saved from about 110 at the factory, but Kentucky Governor Andy Beshear said it would be a miracle if someone was found alive.
BBC North America correspondent Peter Bowes said conditions were extremely difficult for authorities with the Mayfield police station destroyed and firefighters’ equipment damaged.
Tens of thousands of people in the state are also without electricity and water.
At least 12 people died when tornadoes wreaked havoc in other states, including six in an Amazon warehouse in Illinois.
Mr. Biden said he prayed for those who have lost loved ones or are insecure.
“The federal government will do everything possible to help,” he said. “We’ll get through this together.”
The president said teams from the Fema emergency agency would travel to Kentucky on Sunday to offer additional resources, including assistance with temporary housing for people whose homes have been destroyed or badly damaged.
Emergency funds are also available for the other affected states – Missouri, Arkansas, Illinois, Tennessee and Mississippi – if they needed them, he said.
Governor Beshear has declared a state of emergency in Kentucky.
He said Mayfield and everywhere along the 227 mile (365 km) stretch of the tornado – almost all in Kentucky – had been devastated.
“It’s indescribable, unlike anything I’ve ever seen,” he said. “You can see parts of industrial buildings, roofs or sightings in trees when trees are lucky enough to stand.
“Huge metal piles that have been bent in half, if not broken, buildings that are no longer there, huge trucks that have been picked up and thrown. And unfortunately far too many houses, which were probably people, completely destroyed. “
“There are many families who need your prayers,” he added.
Deaths have been reported in several counties, but the loss of life at the Mayfield factory could exceed any tornado event in any single location in the state’s history, the governor added.
Mr Beshear said the factory was “in a very dire situation” as the difficulty of searching the collapsed building was made worse by drums leaking corrosive fluid.
Other incidents in Kentucky included a train derailment in extreme winds in Hopkins County.
Local officials also described how two children were reported missing during a tornado but were then found in a bathtub that had been dragged outside by the wind.
The Amazon warehouse in Edwardsville, southern Illinois, was damaged by another tornado, officials said.
Illinois Police Chief Mike Fillback said six people had died and it was still unknown how many workers were missing.
Tennessee State Emergency Management Agency spokesman Dean Flener said three people were killed in the state’s northwest. Further details were not given.
Two people were killed in Arkansas, one in a nursing home after it was partially collapsed. One fatality has been confirmed in Missouri.