At least 23 people, some children, have been killed after tornadoes struck Lee County in eastern Alabama, authorities said.
Sheriff Jay Jones told the Associated Press the toll could rise, but rescue efforts had to stop until dawn due to the danger of searching in the dark.
“The challenge is the sheer volume of the debris where all the homes were located,” he told CNN.
It is unknown how many are injured. Authorities have blocked off the area.
The National Weather Service (NWS) classified the first tornado to strike as at least an EF-3 – meaning winds of up to 165 miles per hour (266km/h) – carving a path at least half a mile (0.8km) wide.
Authorities say they are still working to identify the victims and the injured. Family members reportedly said one of the victims was an eight year old in the town of Beauregard.
“We’ve never had a mass fatality situation, that I can remember, like this in my lifetime,” Lee County coroner Bill Harris reportedly said.
Rescuers have yet to reach some areas, he said, and coroners from other parts of the state were coming to help.
East Alabama Medical Centre said it was treating more than 40 people injured by the extreme weather, and was expecting more.
Sheriff Jones described the damage as “catastrophic”. PowerOutage.US says there are about 4,000 customers without power across Alabama, with about 2,000 of those in Lee County.
Cold weather is forecast for the region after the tornadoes, with temperatures predicted to drop to near freezing.
Alabama meteorologist Eric Snitil tweeted that there were more deaths in Lee County in one day due to a tornado than in the entirety of the US in 2018.
Tornado watches were in effect in parts of Alabama and in neighbouring Georgia on Sunday evening.
The extreme weather initially cut off electricity for 21,000 Georgia Power customers, according to a company spokeswoman, and tore down trees and destroyed homes.
Alabama Governor Kay Ivey posted on Twitter to warn residents there could be more extreme weather to come.
“Our hearts go out to those who lost their lives in the storms that hit Lee County today,” she wrote.
President Donald Trump also tweeted, asking people to “Please be careful and safe”.
And Apple CEO Tim Cook, who was born in Mobile in the southwest of the state, said he was “devastated” by the news, saying Lee County was “a place close to my heart”.
The National Weather Service said it would send three survey teams to assess the damage caused by tornadoes across the state on Monday.
Schools in the Lee County will be closed on Monday, authorities said, although this will not include school districts in Opelika and Auburn within the county.