Before the disaster that swept through the town of Joplin, the previous worst killed 116 people in Flint, Michigan, in 1953.
More deaths have resulted from outbreaks of multiple tornadoes. On April 27, a pack of twisters roared across six southern states, killing 314 people, more than two-thirds of them in Alabama.
Sunday’s tornado ripped through the heart of Joplin, a working-class city of 50,000 people, slamming into the St John’s Regional Medical Centre. The hospital confirmed that five of the dead were patients – all of them in critical condition before the tornado hit. A hospital visitor was also killed.
The tornado destroyed possibly “thousands” of homes, fire chief Mitch Randles said, and levelled hundreds of businesses.
President Barack Obama is to visit Missouri on Sunday to meet people whose lives have been turned upside down by the twister.
He vowed to make all federal resources available for efforts to recover and rebuild.
“The American people are by your side,” he said. “We’re going to stay there until every home is repaired, until every neighbourhood is rebuilt, until every business is back on its feet.”