The death toll from tornadoes that ripped through Oklahoma has risen to nine and includes two children, authorities said Saturday, less than two weeks after the US state was hit by a monster twister.

The Oklahoma Medical Examiner's Office announced the new fatalities and said five of the victims have not been identified.

The tornadoes hit late Friday and the Oklahoma Department of Emergency Management said that a mother and child were killed as they traveled in their car on an interstate highway.

It was not immediately clear how many people were injured by the series of storms that struck the area around Oklahoma City. The Oklahoman newspaper's website reported that at least 87 people were treated in local hospitals.

As the extent of the devastation became clear Saturday, cleanup efforts were complicated by severe flooding caused by downpours that drenched the region overnight.

Officials from hard hit Canadian County, speaking on CNN, said crews were working to assess and restore "washed out" places.

Reports said five twisters had struck the area around Oklahoma City, with winds of up to 90 miles (145 kilometers) per hour, accompanied by very large hail. Flash floods also hit the area, the Tulsa World newspaper said.

Photos showed streets looking like rivers, with stranded cars submerged in water as high as their door handles in some places.

The National Weather Service's Storm Prediction Center in Norman, Oklahoma warned that the severe weather was shifting eastward Saturday, with the Ohio and Mississippi River Valleys set to receive the bulk of the storms.

Friday's storms were far less damaging than the tornado that hit the Oklahoma City suburb of Moore nearly two weeks ago that left 24 dead and demolished large swathes of the town with winds above 200 miles (322 kilometers) per hour, affecting a total of 33,000 people.

With an average of 1,200 tornadoes per year, the United States is the most hurricane-prone country in the world. They are particularly prominent in the Great Plains states of Kansas, Oklahoma and Texas, as well as in Florida.

Mother and child among 5 killed in Oklahoma tornadoes
Chicago (AFP) June 01, 2013 - Tornadoes killed at least five people and wreaked massive damage Friday in Oklahoma near the battered city of Moore, still recovering from a monster twister less than two weeks ago.

The Oklahoma Medical Examiner's office confirmed the death toll, but said it had no further details on the victims.

But the Oklahoma Department of Emergency Management confirmed a mother and child were killed as they traveled in their car on Interstate 40 near Oklahoma City.

At least 12 people were injured in the storm, with that number expected to rise, the agency told AFP. Media reports, citing local hospitals, put the injury toll at 39 or even higher, but officials said it would take time to get an accurate count.

"We will have to wait for flood waters to recede before we get out to assess damage," spokeswoman Keli Cain said, adding that if the rain continued, that could take several days.

Several people were hurt in crashes and overturned vehicles, including semi-trailers and trailers, Oklahoma Highway Patrol spokeswoman Betsy Randolph told NBC television affiliate KFOR.

"It is a heartbreaking situation," Randolph said.

While storms continued to pass through the state, Cain said, "they're much less severe." However, "there's still a lot of flooding in areas," she said, warning residents to stay off roads and avoid high water.

Weather forecasters lifted an emergency for parts of the tornado-prone midwestern state, though flash flood warnings, severe thunderstorm watches and tornado watches remained in place.

Photos showed streets looking like rivers, with stranded cars submerged in water as high as their door handles in some places.

National Weather Service's Storm Prediction Center in Norman, Oklahoma warned the severe weather was shifting eastward Saturday, with the Ohio and Mississippi River Valleys set to receive the bulk of the storms.

"Large hail, damaging winds and isolated strong tornadoes are all possible," the weather center said on its website.

In Missouri, which borders the Mississippi River, Governor Jay Nixon declared a state of emergency Friday night, urging "Missourians to closely monitor weather conditions, so they can take shelter or move to higher ground if needed."

Reports said five twisters had struck the area around Oklahoma City, with winds of up to 90 miles (145 kilometers) per hour, accompanied by very large hail.

Flash floods also hit the area, the Tulsa World newspaper said.

One large tornado touched down west of Oklahoma City, the state's biggest city, news reports said.

Oklahoma Governor Mary Fallin said earlier she was "heartsick" that Moore, struck by a powerful tornado on May 20, could be hit once again.

KFOR television, meanwhile, reported extensive damage around the cities of El Reno and Yukon. More than 170,000 people were said to have lost power in the Oklahoma City metro area.

Officials for Oklahoma City's Will Rogers World Airport said on Twitter that passengers had been evacuated to an underground tunnel and that flights in and out had been suspended.

The airport was re-opened by around 3:30 am local time (0830 GMT), but all early departures had been cancelled, they said.

Friday's storms were far less damaging than the Moore tornado nearly two weeks ago that left 24 dead and demolished large swathes of the town with winds above 200 miles per hour, affecting a total of 33,000 people, .

With an average of 1,200 tornadoes per year, the United States is the most hurricane-prone country in the world. They are particularly prominent in the Great Plains states of Kansas, Oklahoma and Texas, as well as in Florida.