At least 16 people were killed across the U.S. when a powerful tornado swept through Texas and Oklahoma, NBC News reported.

From the southern plains to the Midwest, an estimated 138 million people were threatened by severe weather. Tornado warnings have been issued for parts of Illinois, Missouri, Arkansas, Mississippi, Indiana, Kentucky, Louisiana and Tennessee.

The storms reduced buildings to rubble, disrupted power and caused more than 100 wildfires. Butler County Emergency Management said the death toll could rise as search and rescue operations on the ground continue.

Millions of people remain at risk as the storm makes its way through the region.

More than 300,000 customers in the Midwest and South are without power, according to the company. PowerOutage.us , an organization that collects real-time data on power outages.

About half a dozen tornadoes were reported in the state of Missouri alone.