An exceptional number of tornadoes have ravaged parts of Oklahoma and nearby Great Plains states, leaving at least five dead, authorities and local media said Sunday.

After 78 tornadoes were reported Friday, mostly in Nebraska and Iowa, a separate weather system Saturday brought 35 tornado reports from northern Texas and Oklahoma to Missouri, the National Weather Service (NWS) said.

The storms dumped up to seven inches (18 centimeters) of rain in some places within hours, and meteorologists warned of a continuing danger of extreme weather Sunday, including flash flooding, hail and more tornadoes.

Sulphur, in south-central Oklahoma, was particularly hard hit late Saturday, with videos and images posted on social media showing scores of buildings ripped apart.

A postal truck sat precariously atop wreckage of a building and trees, one video showed, with cinderblocks and wood beams strewn everywhere. Rescue crews went house to house and vehicle to vehicle in search of victims or survivors.

Oklahoma Governor Kevin Stitt, speaking Sunday from the hard-hit small town, said four people had been confirmed dead across the state.

The damage in Sulphur, where one woman died when the tornado rolled through at nighttime, was the worst he's seen in his six years in office, he told a press conference.

"It seems like every business in downtown has been destroyed now," he said, adding "thank goodness... there wasn't a lot of people here at 10:30 at night."

At least two people died in the Oklahoma town of Holdenville, the state's Department of Emergency Management (OEM) said, with local media reporting a four-month-old baby was among the victims.

OEM said a fourth person died on a highway in Marietta, where videos broadcast by local media showed several battered cars along the side of the road, with two semi-trucks overturned and a nearby warehouse ripped open.

The National Weather Service said a preliminary investigation had confirmed tornadoes in Sulphur and Marietta were at least EF-3 on the five-level Enhanced Fujita Scale, meaning gusts above 136 miles (218 kilometers) per hour.

Stitt has declared a 30-day state of emergency to expedite aid to 12 of the hardest-hit counties, and said he was in contact with federal authorities for assistance.

A man in Iowa who was injured in a tornado Friday later died in hospital, his family told local outlet KETV NewsWatch 7.

More than 25,000 homes in Texas and more than 19,000 in Oklahoma were without power as of Sunday afternoon, the poweroutage.us website reported.

The region is known for the frequency and power of the tornadoes that rumble through every spring. But having separate major outbreaks in succeeding days is exceedingly rare, meteorologists said.