WASHINGTON - Powerful tornadoes killed at least 18 people as they flipped cars, ripped up homes and uprooted trees across south-central United States, emergency officials reported Monday.Rescuers worked through the night using searchlights in blacked-out areas as they sifted through mountains of rubble searching for survivors.Forecasters warned the twisters would continue to threaten much of the region through Tuesday.The Arkansas Department of Emergency Management said that 15 people were killed when tornadoes touched down on Sunday, while an official with the Oklahoma Emergency Management Agency said there were at least two tornado victims in the state. Local media reported another fatality in the state of Iowa. In far-away Manila, President Barack Obama, who is on a tour of Asian nations, offered condolences and promised federal government aid. “I want everybody to know that your country will be there to help you recover and rebuild as long as it takes,” he said. “It’s chaos right now,” the mayor of the Arkansas town of Vilonia, James Firestone, told CNN late Sunday as emergency crews clawed through the debris overnight in some of the hardest-hit areas.The central part of the town of 4,000 “seems like it’s completely levelled. There’s a few buildings partially standing, gas lines spewing. Fire lines down. We’ve had some casualties.”Firestone said that police and firefighters from nearby cities as well as National Guard troops were heading to Vilonia.Twisters also devastated large sections of the town of Mayflower, population 2,300, just northwest of the Arkansas state capital Little Rock.Pictures of tornado damage posted by Arkansas TV station THV 11 showed big-rig trucks crushed like empty cans, homes violently ripped in half, and whole residential blocks leveled and reduced to rubble.Officials said that parts of Interstate 40, a major east-west highway across the United States, was closed due to debris and overturned vehicles in the Mayflower area.Two regional utility companies, Entergy and First Electric Cooperative, said that more than 15,000 customers were in the dark.Firestone later told CNN that emergency sirens blared some 45 minutes before the tornados struck, warning residents to head to shelters and likely saving lives.Sadly, Vilonia has seen this before: the town was struck by a tornado that took “almost the same path” three years ago, Firestone said.“It’s been a truly awful night for many families, neighborhoods and communities, but Arkansans always step up to help each other recover,” Arkansas Governor Mike Beebe wrote on Twitter.The full impact of the storm and its toll will likely not be known until after sunrise at around 1130 GMT.Some homes were uprooted from their foundation. In Iowa, the tornado also dumped heavy rain, snapped trees and lifted the roof off a medical center in the town of Oskaloosa.In Oklahoma, a powerful twister struck the town of Quapaw.“There have been numerous homes and buildings damaged and some destroyed,” Keli Cain of the Oklahoma Emergency Management Agency told AFP.She cited local emergency officials as saying that a fire station was destroyed and there was damage to the northern part of the town.Dozens of homes were also reported destroyed in nearby Kansas, though state officials have reported no fatalities.The National Weather Service warned of a severe weather threat across the central and southern United States over the next days.