Valley View, Texas - At least 15 people were killed across the central US as tornadoes and other extreme storms hit several states including Texas, Arkansas, and Oklahoma, officials said Sunday.

Rescue efforts were ongoing and hundreds of thousands of people were without power after the storms struck the Southern Plains region beginning late Saturday.

In Texas, Cooke County Sheriff Ray Sappington told a press conference that seven people were dead after a tornado ripped through the Valley View area, north of Dallas, according to local ABC affiliate WFAA.

Search and rescue operations were ongoing.

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Texas Governor Gregg Abbott signed a decree for four counties that frees up money and staff to help people whose homes were damaged or destroyed.

The twister destroyed homes and a gas station, and overturned vehicles on an interstate highway.

Sappington called the damage "pretty extensive," in an interview with The Weather Channel.

In Oklahoma, at least two people were dead after a tornado hit Mayes County late Saturday, the county head of emergency management Johnny Janzen told the Fox News affiliate in Tulsa.

And in Arkansas, five people were killed in storms in the early hours of Sunday, authorities confirmed to local ABC station KATV. Power lines and trees were downed, and some areas experienced flash flooding.

Another death was reported in Louisville, Kentucky, Mayor Craig Greenberg said.

As far north as Indiana, the start of the Indianapolis 500 was delayed for four hours Sunday due to storms in the area, with fans asked to exit the bleachers and seek shelter. A crowd of 125,000 was expected for the event.

As the storm system moved across the country, nearly 490,000 customers were without power in states stretching from Texas up to Kansas and east to Ohio and Kentucky, according to the website Poweroutage.us.

Tornado alerts were still active in several places.

The death toll from tornadoes and other extreme weather that buffeted the central US over the weekend climbed to 21 on Monday as the severe storms threatened to roil parts of the East Coast.

Storms were still likely to produce damaging wind and hail as they pushed eastward Monday amid Memorial Day weekend, as well as possibly produce "isolated tornadoes" in the eastern Mid-Atlantic, the National Weather Service said.

Eight people died in northwest Arkansas, four died in Kentucky, seven died in Texas, and at least two have been killed in Oklahoma.