At least 70 people are thought to have been killed in Kentucky after tornadoes ripped across the South and Midwest of the US on Friday.
Kentucky Governor Andy Beshear estimated the toll would 'exceed 50' but could reach 100 after 'some of the worst tornado damage' the state has experienced 'in a long time'.
'It's been one of the toughest nights in Kentucky's history,' Beshear told a news conference early on Saturday. 'I fear that there are more than 50 dead in Kentucky... probably closer to somewhere between 70 and 100, it's devastating.'
A spokeswoman for the state's emergency management service said that rescue officials had not confirmed figures for deaths or injuries as of early Saturday.
Beshear added the town of Mayfield had been devastated and said the damage was 'some of the worst we've seen in a long time.'
At least one person was killed and five were injured when a tornado shredded the roof of a nursing home in Monette in northern Arkansas and another person killed in Missouri.
A further three people were killed in the severe weather in Tennessee, Dean Flener, spokesman for the state's Emergency Management Agency said.
Elsewhere, in Illinois, authorities said many people were trapped after a roof partially collapsed at an Amazon.com Inc warehouse near St. Louis late on Friday, after tornadoes and strong storms blew through the area.
Speaking at a press conference early Saturday morning, Beshear said: 'As of 4:45am [CST] 56,854 Kentuckians are without power. I have been personally over at the emergency operations centre since about 1am overseeing our response and hearing and absorbing the difficult news in real time.'
He said: 'Before midnight, I declared a state of emergency. I've activated the national guard. We are deploying 181 guardsmen including search and extraction and debris clearing folks.'
He said search and rescue officials had been working through the night to pull people from the rubble.
Meanwhile one person was killed in Missouri, in rural St. Charles County, when buildings collapsed near the town of Defiance. And in Arkansas, one person was killed in a nursing home in Monette, according to KARK.
'It happens quick but apparently there was a little bit of time with tornado sirens going off.'
Some residents were found in the basement 'and were prepared for this,' he said.
Melissa Moon, a reporter with WREG3, tweeted a photo of the severely damaged Monette Manor nursing home, with what appeared to be a mangled bed in the parking lot.
The exact speed of the tornadoes were not yet known on Friday night, and it was unclear how many there had been. CNN reported that there were 19 tornadoes that hammered the five state.
Some reports suggested that the record for the longest single tornado - 219 miles - could have been broken, and that it had crossed four states, breaking another record.
Observers speculated that many of tornadoes were at four or even five on the Enhanced Fujita (EF) scale.
Wind speeds of between 136 and 165mph are found in EF4 tornados, and of 200mpg in EF5 tornados.
'This tornado is so powerful it literally broke Radarscope velocity,' tweeted @WxAtlantic.
'The delta velocity is so high that the couplet just folded over on itself.'
Almost 300 miles to the north of Monette, Southern Illinois Fire Incidents confirmed a 'mass casualty incident' at the Amazon warehouse in Edwardsville and said more than 20 units of emergency responders were attending the scene.
'About a third of the warehouse is torn down and damaged from either straight by line winds or tornado,' tweeted Jenna Rae, with Illinois's KMOV station.
One woman said she was speaking to a family member inside the warehouse as the storm hit.
'He was on the phone with me while it was happening,' Aisha White told KMOV.
'The tornado was hitting the back of the building, the trucks were coming in, I told him to jump out the truck and duck.
'We watched the building go up, stuff hitting the cars, I told him I was on my way.'
J.B. Pritzker, governor of Illinois, tweeted: 'My prayers are with the people of Edwardsville tonight, and I've reached out to the mayor to provide any needed state resources.'
Richard Rocha, an Amazon spokesman, said: 'The safety and well-being of our employees and partners is our top priority right now.
'We're assessing the situation and will share additional information when it's available.'
And in Kentucky, the town of Mayfield - home to 10,000 people - was hard-hit.
Buildings were left without their roofs; large trees were uprooted; glass in store fronts was shattered, and debris scattered on the ground.
The courthouse, built in 1888, was devastated, with the tower sheared off the building and the roof lifted off.
The wall of a candle factory collapsed, trapping people inside and leaving them begging for help.
'We got hit by a hurricane - I'm at work in Mayfield, and we are trapped,' said Kyanna Lou, broadcasting a Facebook Live from the dark, with a woman sobbing behind her and muffled shouts and moans.
'Please, y'all. Get us some help. We're at the candle factory in Mayfield.
'Please, send us some help. The wall is stuck on me. Nobody can get to us, y'all please. Pray for us.'
She added: 'We were all in the safe shelter place, the whole building fell. We are stuck.'
Crews at the scene reported the Monette nursing home had partially collapsed, with five others injured and as many as 20 people trapped, according to Craighead County Judge Marvin Day.
Initial reports said two were dead but within two hours, the building had been cleared and everyone initially believed to have been inside had been accounted for, Day said.
'It looks like it's pretty much destroyed,' Day said of the building.