At least 23 people, including children as young as six, have been killed and more are missing after at least two tornadoes struck in Alabama on Sunday, causing "catastrophic" damage, a county sheriff has said.

Crews searched a trail of destruction several miles long before pausing efforts overnight as conditions became too dangerous, Lee County Sheriff Jay Jones said. "The challenge is the sheer volume of the debris where all the homes were located," Jones told CNN. "It's the most I've seen that I can recall."

East Alabama Medical Center said it had received more than 40 patients as a result of the tornado and more were expected. The county coroner, Bill Harris, said none of the victims had been formally identified yet but believed the youngest victim was six.

He told how firetrucks were drafted in to take victims out of the danger zone unreachable by ambulances and that he had to use his paramedic skills at the fire station under more help arrived.

"This is a day of destruction for Lee County, we've never had a mass-fatality situation that I can remember like this in my lifetime," Harris said. Some areas were still inaccessible, he added, suggesting the death toll could still rise.

Sunday's destruction was caused by a severe weather system that crossed the US south-east in the afternoon, sparking tornadoes as it headed towards the Atlantic seaboard.

Donald Trump urged people in the region to stay safe, tweeting that the tornadoes and storms "were truly violent and more could be coming".

Earlier, Harris, when confirming two deaths in Beauregard, Alabama, said: "We've still got people being pulled out of rubble. We're going to be here all night."

Rita Smith, a spokeswoman for Lee County Emergency Management Agency said: "We've got about 150 first responders out there. They are doing a phenomenal job. Sadly, we know that we have two known confirmed fatalities and many, many injuries."

She said multiple homes had been destroyed or damaged in Beauregard, a community about 60 miles (95km) east of Montgomery, the state capital.

The East Alabama Medical Center in Opelika said in a statement that it was treating more than 40 patients as a result of the tornado and expects to receive more. Some patients have been sent to other hospitals, it added.

The storm has left more than 10,000 customers without power, the Birmingham News reported, and temperatures looked set to fall to near freezing overnight. "Colder air will sweep into the south-east behind the severe weather with temperatures dropping into the 30s [1C] southward to central Georgia and across most of Alabama by Monday morning," AccuWeather meteorologist Kristina Pydynowski said. "Those without power who rely on electric heat need to find ways to say warm."

Televised broadcast news footage showed smashed buildings with rooftops blown away, cars overturned and debris everywhere. Trees all around had been snapped bare of branches.

The National Weather Service said it had confirmed a tornado by radar that toppled trees in a Florida Panhandle county, halting traffic on a stretch of Interstate 10 in one direction because of debris.

Meteorologist Don Harrigan with the Tallahassee office of the weather service said the tornado hit about 5.45pm on Sunday in Walton County and other tornadoes had been confirmed on radar in Geneva County in south-east Alabama, just across the line from north Florida, and in Henry County, Alabama.

Harrigan said a squall line moving across the south-east was entering an area of strong, low-level winds conducive to forming tornadoes. He said the threat of more tornadoes would continue for several hours as the storm system headed toward the Atlantic seaboard.

A tornado watch was in effect for much of Georgia, including Athens, Augusta and Savannah. The tornado watch also covers a large area of South Carolina, including the cities of Charleston and Columbia.

With Associated Press

The monster tornado that cut through Alabama Sunday was the deadliest in the U.S. since 2013, according to the National Weather Service.

Twenty-three people died in the afternoon twister that hit Lee County in the southeast part of the state. The number of fatalities is the greatest in the U.S. since an EF-5 tornado killed 24 people in Moore, Oklahoma on May 20, 2013.

The death toll in Alabama may rise, officials said, as search, rescue and recovery efforts continues. Three children, ages 6, 9 and 10 years old, are among the fatalities.

The NWS in Birmingham said preliminary assessments show the Lee County storm was likely an EF-4 with winds at around 170 mph. The Moore tornado had maximum winds of some 210 mph.

The Lee County deaths are more than double the 10 tornado-related fatalities that occurred in 2018, which had the fewest for a calendar year on record, according to weather.com. The tornado is believed to have been about a mile wide and spanned about 24 miles in length, leaving a path of destruction as it made its way through Alabama, Georgia and South Carolina.

Sunday's deaths are the first tornado-related fatalities in Alabama since November 2016.

Three other tornadoes were also reported in Sunday: south of Tuskegee in Macon County into Lee County; County Road 79 in Barbour County; and Eufaula in Barbour County. All were at least EF-1 tornadoes.

The Eufaula storm was likely on the high-end of the EF-1 scale with tornado damage confirmed northwest of Eufaula, mostly concentrated on County Road 79, south of Highway 82. EF-1 tornadoes have winds of between 86 and 110 mph.

The deadliest day in Alabama came on April 17, 2011 when 238 people in the state were killed by a wave of tornadoes. The total death toll on that day across the U.S. topped 324, making it one of the deadliest ever recorded.