US President Barack Obama meanwhile declared a “major disaster” in Oklahoma and ordered federal aid to supplement local recovery efforts.
On Twitter, the National Weather Service gave the tornado a preliminary rating of EF-4, indicating that it packed winds of 166 to 200 miles per hour (267-322 km/h) — more severe than a category five hurricane.
In downtown Oklahoma City, tornado sirens went off at least three times and the Interstate 35 highway — a busy north-south artery through the American heartland — was closed to all but emergency vehicles.
In Moore, live images from KFOR showed people wandering among the debris and even a couple of untethered horses from a local stable that somehow managed to survive the punishing storm.
“I had no idea it was coming,” said a stable worker, who told how he survived the “unbearably loud” twister by taking cover in one of the stalls.
Monday’s tornado followed roughly the same track as a May 1999 twister that killed 44 people, injured hundreds more and destroyed thousands of homes.
Tornadoes frequently touch down on Oklahoma’s wide open plains, but Monday’s twister struck a populated urban area and raised fears of a high casualty toll.
Because of the hard ground, few homes are built with basements or storm shelters in which residents can take cover.
Oklahoma City lies inside the so-called “Tornado Alley” stretching from South Dakota to central Texas, an area particularly vulnerable to tornadoes.
On Sunday, a powerful storm system churning through the US Midwest spawned tornadoes in Iowa, Kansas and Oklahoma, destroying homes and killing at least two people, US media reported.
Fallin had already declared a state of emergency for 16 Oklahoma counties due to the tornado threat on Sunday, and added five more on Monday after the storms hit her capital.