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.
Medical centre devastated
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.
The injuries could go beyond those sustained directly as a result of the tornado.
"There are hundreds of people walking through the streets," said Hendren.
"The emergency officials here have been so busy dealing with the search that they're having a hard time keeping people from combing through the wreckage, and, of course, that remains dangerous."
More tornadoes possible
The National Weather Service (NWS) predicted a 10 percent chance of tornadoes in parts of Texas, Oklahoma, Arkansas, Kansas, Missouri and Illinois.
It said parts of four other states - Wisconsin, Indiana, Michigan and Iowa - had a five percent risk of tornadoes.
The area at greatest risk includes Joplin, Missouri, which on Wednesday will mark two years since a massive tornado killed 161 people.
Emergency crews searched the rubble of Plaza Towers Elementary School for two dozen missing children, Oklahoma Lieutenant Governor Todd Lamb said.
Witnesses said Monday's tornado appeared more fierce than the giant twister that was among the dozens that tore up the area on May 3, 1999, killing more than 40 people and destroying thousands of homes.
Survivor Ricky Stover relived his terror as he surveyed the devastated remains of his home.
"We thought we died because we were inside the cellar door," Stover said. "It ripped open the door and just glass and debris started slamming on us and we thought we were dead."
The NWS assigned the twister a preliminary ranking of EF4 on the Enhanced Fujita Scale, meaning the second most powerful category of tornado with winds up to 320kph.
Oklahoma activated the National Guard, and the US Federal Emergency Management Agency activated teams to support recovery operations and coordinate responses for multiple agencies.
The latest tornado in Oklahoma came as the state was still recovering from a strong storm on Sunday with fist-sized hail and blinding rain.
Two men in their 70s died in the storm, including one at a mobile home park on the edge of the community of Bethel Acres near Oklahoma City, said Keli Cain, a spokeswoman for the Oklahoma Office of Emergency Management.
Thirty-nine people were injured around the state as storms toppled trees and tore up rooftops, she said.