Powerful tornados ripped through Oklahoma, with dozens believed to have been killed in winds of up to 200mph, including at least 20 children at an elementary school.
As many as 26 tornadoes were reported in Oklahoma, Kansas, Illinois and Iowa, according to the National Weather Service, with 91 people confirmed dead in Oklahoma thus far.
The storm laid waste to scores of buildings in Moore, a community of 41,000 people about 10 miles south of Oklahoma City. Street after street lay in ruins, homes were crushed into piles of broken wood and cars and trucks were left crumpled on the roadside.
The Governor declared a state of emergency in 16 counties and President Barack Obama declared a major disaster area in Oklahoma, pledging his government would provide federal aid in addition to state resources.
Emergency services are still searching the rubble of the devastated Plaza Towers Elementary School, which took a direct hit in the powerful storm, its roof ripped off and walls flattened.
A water pipe is believed to have burst at the school and a number of the children could have drowned after becoming trapped in the debris.
Oklahoma's Lt Gov Todd Lamb told the BBC: "The school was flattened. The walls were pancaked in. There have been some bodies recovered from that school and it's absolutely horrific and devastating."
Worst hit was the town of Moore, street signs disappeared under flying detritus, lights went out, and houses, schools and a hospital were completely obliterated, Reuters reported.
"The whole city looks like a debris field," Glenn Lewis, the mayor of Moore, told NBC.
"It looks like we have lost our hospital. I drove by there a while ago and it's pretty much destroyed," Lewis said.
Brady Williams, from the town of Norman, and his girlfriend drove south to witness the tornado’s damage and assist with rescue efforts.
“We soon found ourselves in a neighbourhood that was absolutely devastated by this tornado,” he told HuffPost by email. “Nearly every single house was turned into a pile of rubble as far as the eye could see.”
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Governor Mary Fallin, speaking to Fox News on Monday night, described her reaction to the tragedy at Plaza Towers Elementary School.
"Your hearts just break for the parents who don't know the answer to the question about whether their child is safe," Fallin said. "I've been through a lot of disasters in Oklahoma, but to see this sight, to see how many areas are devastated, is just absolutely incredible. It's just beyond even expressing words."
She said that five different schools in the area were hit by the tornado.
The tornado, she said, was "by far, probably, the worst I've ever seen."
The Red Cross is supporting first responders and is providing shelter, food, distributing relief items and clean-up supplies.
“These are dangerous storms and we urge people to monitor the situation closely and be alert for severe weather warnings in their community,” said Trevor Riggen, vice president of Disaster Operations and Logistics for the Red Cross.