The highest level tornado warning has been issued in Texas and Oklahoma by the U.S. National Weather Service's Storm Prediction Center.
Forecasters warned of a "major severe weather outbreak" on Monday over parts of northwest Texas and western and central Oklahoma.
Around six million people need to brace for possible large tornadoes along with hail, flash flooding and hurricane force winds.
The first tornado touched down on Monday afternoon in a rural area of Texas.
The federal Storm Prediction Center advised people in the region to have a plan for seeking shelter if a storm approaches.