Tornadoes killed at least 14 people in the US Midwest, including two children, officials said Saturday, as flooding hampered cleanup efforts in Oklahoma, still recovering from a monster twister that struck last month.

Friday night's storms battered areas in and around Oklahoma City with high winds, heavy rain and hail.

The Oklahoma Medical Examiner's Office announced nine fatalities in the state and said five of the victims have not been identified, while the sheriffs' offices in towns east of Oklahoma City confirmed two other people had died.

In Missouri, authorities said three people died from severe flooding in the wake of the storms.

Streets turned into rivers, with stranded cars submerged in water as high as their door handles in some places. CNN said a massive sink hole off a major road developed due to the deluge, halting traffic.

Two of the victims have not been identified. A mother and child perished as they traveled in their car on an interstate highway, according to the Oklahoma Department of Emergency Management.

The National Weather Service's Storm Prediction Center in Norman, Oklahoma, said the first tornado developed around the city of El Reno before moving into parts of Oklahoma City, spawning others.

Local broadcaster KOCO reported that 77 people had been admitted to hospitals with storm-related injuries.

Missouri Governor Jay Nixon, who toured the widespread devastation to assess the damage, urged residents of his state to avoid walking or driving through flooded areas.

"Missouri has been hit by several rounds of severe storms in the past few weeks, and last night's dangerous weather follows several days of heavy rain," he said in a statement.

"Because many streams and rivers are overflowing their banks, we will need to stay vigilant in both monitoring and responding to flooding across the state as well. This remains a dangerous situation," he said.

As the extent of the devastation in Oklahoma became clear, the work of cleanup crews was complicated by downpours that drenched the region overnight.

"We're going to get through this again," Governor Mary Fallin told CNN.

Officials from hard-hit Canadian County told reporters that crews were working to assess and restore "washed out" places.

A trailer park in Oklahoma City was among areas evacuated by boat, raft and Humvee, according to KOCO.

As the storms approached, Oklahoma City's Will Rogers World Airport evacuated passengers to an underground tunnel and suspended incoming and departing flights. It re-opened around 3:30 am (0830 GMT), but all early departures had been canceled, officials said.

Power company OG&E, meanwhile, reported 74,093 outages by mid-afternoon Saturday and the American Red Cross has opened shelters for those in affected areas.

The National Weather Service warned that the severe weather was shifting eastward, with the Ohio and Mississippi River Valleys set to receive the bulk of impending storms.

Friday's twisters were far less damaging than the tornado that hit the Oklahoma City suburb of Moore nearly two weeks ago.

That massive funnel cloud left 24 dead in its wake and demolished large swathes of the town with winds above 200 miles (322 kilometers) per hour. In total, some 33,000 people were affected.

In Iowa, Governor Terry Branstad issued a disaster proclamation for several counties in response to widespread flooding.

Meanwhile, a tornado ranking an EF-2 on the Enhanced Fujita scale damaged about 50 buildings in Illinois, including a high school in Gillespie, Illinois which had its roof sheared off by 115-mile per hour winds.

"We had houses twisted off foundations, houses with roofs taken off," said James Pitchford, an emergency management official in the US heartland state.

The United States is hit by an average of 1,200 tornadoes each year. They are particularly prominent in the Great Plains states of Kansas, Oklahoma and Texas.

Saturday also marked the official start of the US hurricane season, which some forecasters say will bring more extreme weather from the Atlantic, triggered by warmer than usual water temperatures.