Rescuers and residents searched house to house Saturday a day after tornadoes killed at least 37 people and injured hundreds, tearing across the US heartland and virtually wiping out communities.

Even as stunned Americans grappled with the magnitude of the destruction brought by Friday's twisters, the National Weather Service (NWS) issued new tornado warnings for parts of Georgia and Florida, in the country's southeast.

Trucks and trees were upended as deadly funnel clouds ravaged parts of eight states in the US Midwest and South.

The devastating images included a school bus smashed through the wall of a house, trucks thrown into lakes, solid brick homes reduced to rubble and wooden ones smashed into kindling, as well as mobile homes flipped like tin cans.

President Barack Obama called the governors of Indiana, Kentucky and Ohio to offer condolences for the dead and said the federal emergency management agency stood ready to help, the White House said.

Deaths were reported in Indiana, Kentucky, Ohio, Alabama and Georgia as the storm system moved eastward.

In Kentucky, where Governor Steve Beshear's office confirmed 18 fatalities, a total of 13 tornadoes roared across the state, causing damage in 40 counties and knocking out power to 22,000 people.

About 300 injuries have been reported in the state, according to Beshear, who surveyed the damage in the devastated town of West Liberty.

Amateur video aired on CNN showed a gargantuan grey twister churning over West Liberty on Friday, as a woman loudly prayed "Oh God, take it away from us Lord!"

At least 14 people were killed in Indiana, according to Governor Mitch Daniels, who inspected the devastation in Henryville.

"We're not unfamiliar with Mother Nature's wrath out here in Indiana, but this is about as serious as I've seen it in my years in this job," an emotional Daniels, wearing a camouflage jacket, told reporters.

"Lucky it wasn't worse," he said, adding that while early warning systems likely saved lives, it was a "heartbreaking" loss for families.

The high school in Henryville suffered damage, but luckily all the children were evacuated safely and only minor injuries -- some cuts and scrapes -- were reported, said sheriff department spokesman Chuck Adams.

Officials in Clark County, Indiana were scrambling to deal with widespread damage after roads were blocked by fallen trees and debris, and power and phone lines were knocked out.

The hardest hit was Marysville, where the small town has nearly ceased to exist, officials said. "That's the information we have, that Marysville is no longer," US Senator Dan Coats of Indiana told CNN.

Indiana activated 250 members of its National Guard, who used Black Hawk helicopters to reach hard-hit regions. Indiana and Kentucky declared states of emergency.

There were three deaths in neighboring Ohio, including a city councilwoman from the town of Moscow, an Emergency Management Agency official said.

The Gulf Coast state of Alabama reported one death after tornadoes trapped people in rubble, destroyed houses and uprooted trees.

For Brandy Robbins, whose home in Harvest, Alabama was destroyed in devastating tornadoes last year, it was a sickening case of deja vu.

"I realized lightning does strike twice," she told Fox News, standing outside the home she recently rebuilt, only to have it badly damaged once more.

"Unfortunately my kids and I are going to have to rebuild again."

In Georgia, a woman was killed in the city of Alpharetta, north of Atlanta, and tornadoes inflicted severe damage on at least 40 homes and a regional airport west of the city in Paulding County, a spokeswoman for the state's emergency management agency said.

The latest wave of storms comes as people were picking through rubble left behind by a string of twisters across six states that killed 13 people earlier in the week.

The NWS received 83 reports of tornadoes in eight states by Friday evening, bringing the week's total to 133.

More could be on their way as a "particularly dangerous" tornado watch continued into Saturday in four states in a massive storm that also carried golf-ball sized hail.

Some 545 people were killed by tornadoes in 2011, the deadliest season since 1936 and the third worst on record.

This year tornadoes have come early with the mild winter creating the conditions for cold fronts to slam into warmer air.

"We knew it was going to be bad," said Angie Lese, a meteorologist with NWS. "All the ingredients came together for a significant outbreak." (AFP)