* Most deaths in Arkansas and Mississippi
* Worries of more tornadoes as volunteers sift through rubble (Adds size and speed of Monday's storm in Mississippi, threat of more storms)
By Emily Le Coz
TUPELO, Miss., April 29 (Reuters) - At least 34 people were killed in tornadoes unleashed by a ferocious storm system that razed neighborhoods in the southern United States over the last three days and threatened more destruction in heavily populated parts of the South on Tuesday.
In Arkansas and Mississippi, the hardest hit of six states struck by the storm, there have been 27 confirmed storm-related deaths and more than 200 people injured as tornadoes reduced homes to splinters, snapped trees like twigs and sent trucks flying through the air like toys.
Deaths were also reported in Oklahoma and Iowa on Sunday, and Alabama and Tennessee on Monday.
The White House said President Barack Obama declared a major disaster in Arkansas and ordered federal aid to supplement state and local recovery efforts.
Makeshift shelters have been set up for thousands of families forced out of their homes while the National Guard, local police and residents who had lost all their possessions sifted through the rubble looking for more victims.
"People were running around screaming, trying to find their kids. There was nothing left," Melba Reed said as she described the aftermath of a tornado in Louisville, Mississippi, a town of about 7,000 in the central part of the state.
The tornado that ripped through Mississippi's Winston County on Monday packed winds of up to 200 mph (322 kph), according to the Mississippi Emergency Management Agency.
In Tupelo, Mississippi, police went from house to house, searching for victims and trying to seal gas leaks.
In Park Hill, a traditionally black and densely populated neighborhood of Tupelo, the sound of chainsaws reverberated in the air as children joined adults to help remove debris and residents handed out grilled hotdogs on the streets to anyone in need of food.
"The damage is overwhelming, but the outpouring of people to help lend a hand is overwhelming, too," said Denise Hardin, who works at the Tupelo Housing Authority.
'UNDER THE GUN'
More than 2,000 houses and 100 commercial properties have been reported damaged, and there is a risk of worse to come.
A massive area home to tens of millions of people stretching across large parts of the South and into Pennsylvania and Ohio was under some threat from the storm system that spawned the tornadoes, forecasters said.
"We will see tornadoes again today," said Bill Bunting, operations chief at the National Weather Service's Storm Predictions Center in Norman, Oklahoma. "Unfortunately, the areas that are under the gun today are the same ones that were under the gun yesterday."
More storms are expected on Wednesday, potentially threatening northern Florida, southeast Alabama, the Carolinas and parts of West Virginia, Bunting said. There is also some threat to parts of Pennsylvania and Ohio.
Southern and eastern Mississippi as well as central and western Alabama were at greatest risk from tornadoes, damaging winds and hail, he said.
Utility companies said tens of thousands of customers along the path of the storm were without power on Tuesday morning, with the worst outages in parts of Alabama and Georgia.
In western North Carolina, fire department personnel used boats to rescue people from homes and vehicles hit by flash floods during the night.
Residents of Arkansas' central Faulkner County, where most of the damage occurred, sorted through the rubble as they tried to piece their lives back together.
"When you talk about a tornado, it's just in a matter of moments that your whole home is missing, your belongings, your personal effects," said Matt Payne, a volunteer helping in the relief efforts.
(Reporting by Robbie Ward and Emily Le Coz in Tupelo, Mississippi, Curtis Skinner in New York, Colin Sims in Vilonia, Arkansas, Verna Gates in Birmingham, Kevin Gray in Miami, John Peragine in Lake Lure, North Carolina, Tim Ghianni in Nashville, Tennessee and Scott DiSavino in New York; Writing by Jon Herskovitz; editing by Gunna Dickson , Sharon Bernstein and Simon Cameron-Moore)