At least four people died, including a four-month-old baby, and scores were injured in Oklahoma this weekend after dozens of twisters swept the US Southern Plains, while weather alerts on Sunday put more than 7 million Americans under tornado warnings.

Oklahoma Governor Kevin Stitt on Sunday declared a disaster emergency for the state, freeing up more money for first responders and recovery operations.

More than 20,000 customers in the state remained without power as of Sunday evening, according to officials, as emergency personnel worked to clear roads and repair power lines

"Definitely the most damage since I've been governor," Stitt said in Sulphur, one of the hardest-hit communities, on Sunday afternoon as he provided an update on fatalities and damage. Stitt began his first term as governor in 2019.

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In a call with the Oklahoma governor, President Joe Biden offered the full support of the federal government to help with the recovery efforts, the White House said in a statement on Sunday.

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Biden on Sunday declared a major disaster over the severe winter storms that flooded the State of Washington and Kansas, the White House said in a statement. The president ordered federal assistance to supplement recovery efforts in the areas affected by the winter storms.

Storm warnings for high winds, heavy rain and hail also were issued by the National Weather Service on Sunday for more than 47 million people stretching from East Texas north through Illinois and Wisconsin.

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The NWS reported 38 possible twisters hit the area and that the worst of the storms rolled through Central Oklahoma on Saturday into early Sunday morning, spreading into northwest Texas, western Missouri and Kansas.

The Oklahoma Department of Emergency Management said area hospitals reported 100 injuries. Twisters destroyed or damaged dozens of structures, including the hospital in the town of Marietta, although no injuries were reported there.

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More than 20,000 customers in the state remained without power as of Sunday evening, according to officials, as emergency personnel worked to clear roads and repair power lines.