The weather service's preliminary classification of Monday afternoon's tornado was an EF-4 on the enhanced Fujita scale.
Authorities say emergency crews are working to rescue people trapped in Moore, which is southwest of Oklahoma City.
Oklahoma City Police Capt. Dexter Nelson said emergency crews are trying to reach the affected areas.
Plaza Towers Elementary School in Moore took a direct hit, but reports are that children have been pulled out alive.
The suburb of Moore was hit hard by a tornado in 1999. The storm had the highest winds ever recorded near the earth's surface.
