CHICAGO - Tornadoes that ripped through Oklahoma killed nine people, including two children, officials said Saturday, as flooding hampered cleanup efforts in the US state recently ravaged by a monster twister.

The Medical Examiner’s Office revised the death toll upward from five in the wake of Friday night’s storms that battered areas in and around Oklahoma City with high winds and heavy rain.

Five of the victims have not been identified. The Oklahoma Department of Emergency Management said earlier that a mother and child perished 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. Local broadcaster KOCO said on its website that 77 people had been admitted to hospitals with storm-related injuries.

Reports said five twisters had struck 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.

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.

Saturday also marked the official start of a US hurricane season in which some forecasters have said there could be more extreme weather coming in from the Atlantic, triggered by warmer than average water temperatures. As the extent of the devastation in Oklahoma became clear, the work of cleanup crews was complicated 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.  A trailer park in Oklahoma City was among areas evacuated Saturday morning by boat, raft and Humvee, according to KOCO. Photos showed streets looking like rivers, with stranded cars submerged in water as high as their door handles in some places.

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 canceled, they said.

Power company OG&E reported 91,190 electricity outages as of noon Saturday and the American Red Cross has opened shelters for those in affected areas.

The National Weather Service’s Storm Prediction Center 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.

Severe weather was also reported in the US state of Missouri and Illinois, as well as elsewhere in the Midwest.

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