Moore, Okla., an Oklahoma City suburb, was devastated by tornado on Monday afternoon
Video showed a funnel cloud stretching from the sky to the ground, kicking up debris.
A school was among the structures leveled by the twister.
The National Weather Service issued a rare tornado emergency for the Oklahoma City metropolitan area, meaning that significant and widespread damage and fatalities were likely.
The tornado was estimated to be at least two miles wide at one point as it moved through Moore, Okla., in the southern part of the Oklahoma City metropolitan area, CNN affiliate KFOR reported.
The preliminary rating of the tornado was at least EF4 (166 to 200 mph), the National Weather Service said.
The severe weather comes after tornadoes and powerful storms ripped through Oklahoma and the Midwest earlier Monday and on Sunday.
Oklahoma Gov. Mary Fallin declared a state of emergency after tornadoes tore through 16 counties in her state Sunday.
Viewed from the air, the extent of the damage was staggering, said John Welsh, a helicopter pilot for CNN affiliate KFOR in Oklahoma.
Several structures could be seen on fire in Moore, Okla., in the wake of the twister.