A tornado with 320 kilometer-per-hour winds has killed at least 37 people and caused massive destruction in the central U.S. state of Oklahoma, destroying two schools and entire neighborhoods.
The Oklahoma Medical Examiner's Office said the death toll was expected to rise as rescue workers move deeper into the hardest-hit areas.
The 1.6 kilometer wide tornado hit Monday afternoon and destroyed large swaths of Moore, an Oklahoma City suburb, injuring dozens of people, sending debris flying and setting buildings on fire.
Rescue workers have pulled several children alive out of the rubble of the schools.
Oklahoma Governor Mary Fallin deployed 80 National Guard members to assist with search-and-rescue operations.
Fallin also spoke with President Barack Obama, who asked the Federal Emergency Management Agency to provide any assistance she needs.
The severe weather outbreak was expected to spread across other parts of the Plains and the Midwest. An earlier tornado killed two people in Oklahoma Sunday.
The National Weather Service placed parts of five storm-battered states -- Oklahoma, Kansas, Missouri, Arkansas and Texas -- under a tornado watch, meaning conditions are favorable for tornadoes to develop.
The same suburb of Oklahoma City was hit hard by a tornado in 1999. That storm had the highest winds ever recorded near the Earth's surface.