A massive tornado carrying winds of up to 320 kph has torn through the Oklahoma City suburb of Moore, ripping up at least two schools and leaving a wake of tangled wreckage.
At least 50 people are dead, including seven children in a school which collapsed during the storm. Seventy-five children took shelter there from the tornado in the town of Moore. Up to two dozen are still believed to be trapped under the rubble after Monday's 3-km-wide tornado.
President Barack Obama spoke with Oklahoma Governor Mary Fallin, expressing his concern for those who have been affected and promising all available assistance.
At a press conference in Oklahoma City, Fallin said that additional resources have been called in from out of state, including rescue dogs, as "it will be dark pretty soon and we want to do everything that we can to continue to look for those that might be lost in this tragedy."
She encouraged civilians to stay away from the disaster-struck areas and to leave the work of search and rescue to emergency personnel.
Television video showed tracts of homes destroyed, cars tossed about and piled atop one another, and at least one building on fire.
Rescue workers were pulling third-graders from a severely damaged elementary school in Moore, a KFOR television reporter said from the scene, and aerial video showed first responders sifting through the rubble left behind.
The National Weather Service assigned the twister a preliminary ranking of EF4 on the Enhanced Fujita Scale, meaning Monday's was in the second most powerful category of tornado.
"It seems that our worst fears have happened today," said Bill Bunting, National Weather Service meteorologist in Norman, Oklahoma.
The massive twister struck at the height of tornado season, and more were forecast. On Sunday, tornadoes killed two people and injured 39 in Oklahoma.
Witnesses said Monday's tornado appeared to be more fierce than those that tore up the region on May 3, 1999, killing more than 40 people and destroying thousands of homes. That tornado ranked as an EF5, meaning it had winds over 320 kph.
Unknown number of survivors
Officials at two hospitals in the area said they've been treating more than 140 patients, including about 70 children.
"We have many injured and we're just trying to work out how bad it is right now," said a woman who answered the phone at the Moore city manager's office.
Al Jazeera's John Hendren, reporting from Oklahoma City, said the number of injuries remains unknown as the roof of the Moore Medical Centre has "been completely ripped off of the building" and that survivors will 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.
The National Weather Service 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 - have a 5 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.
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.
Several hundred homes and buildings were thought to have been damaged or destroyed and approximately 7,000 customers were left without power in Oklahoma. "There is definitely quite a bit of damage," Cain said.
Oklahoma Governor Mary Fallin declared 16 counties disaster areas.
More than two dozen tornadoes were spotted in Iowa, Oklahoma, Kansas and Illinois, according to the National Oceanic and Atmospheric Administration and local news reports. Hail stones, some as large as baseballs, were reported from Georgia to Minnesota, NOAA said.
The tornado season in the United States had been unusually quiet until last week, when a tornado struck the town of Granbury, Texas, killing six people.