Oklahoma. US President Barack Obama has declared a major disaster in Oklahoma after a 3km-wide tornado packing winds of up to 320kph tore through the state capital killing at least 51 people, including 20 children, and injuring at least 230.The Oklahoma medical examiner’s office said on Tuesday that it was expecting the death toll from Monday’s tornado to rise to 91 people.Officials in the Oklahoma City suburb of Moore said that many people were missing after the tornado tore up at least two schools, trapping two dozen children beneath rubble, and obliterated a hospital, numerous other buildings and vehicles in its path.Moore, which has a population of about 50,000, was strewn with debris, with street signs gone and lights out in the structures left still standing in the most severe of a series of savage storms to hit the state on Sunday and Monday.Obama spoke with Oklahoma Governor Mary Fallin to express his concern and ordered federal aid to help state and local recovery efforts.Fallin told reporters that "hearts are broken” for parents looking for their children.She declared 16 counties disaster areas and deployed the state National Guard and extra police to assist with rescue operations."We’re doing everything we can … to find anyone who might be injured or might be lost,” Fallin said.The Oklahoma medical examiner said 20 of the confirmed deaths were children and the toll was expected to rise.At least 45 of the 230 people known to have been injured were children, according to area hospitals.Rescue teams raced against the setting sun and worked into the darkness in search of survivors throughout the wide swath of devastation, while the dangerous storm system threatened several southern Plains states with more tornadoes.Severe weather was expected through the night from the Great Lakes south to Texas.Al Jazeera’s John Hendren, reporting from Oklahoma City, said the roof of the Moore Medical Centre had "been completely ripped off of the building” and that survivors would have to be taken to nearby towns for treatment.