A massive tornado has ravaged the suburbs of Oklahoma City killing at least 91 people including 20 children and destroying two schools, say US officials.

Moore, a suburb in the south of the city, is the worst affected by the two-mile-wide tornado.

The death toll is likely to increase with more than 120 people, including many children, having been admitted to hospitals for treatment. One of the schools took a direct hit from the tornado.

Winds with speeds of up to 200mph (320km/h) have wreaked havoc on several buildings in the region, trapping people inside. Scores of vehicles have been left mangled by the storm. Officials are still working to assess the damage caused by the twister, which raged for about 45 minutes.

Rescue efforts are likely to continue through the night with more than 200 personnel taking part in the operation.

"I'm speechless. How did this happen? Why did this happen? How do we explain this to the kids? How are they going to wake up tomorrow and everything's missing - the school, these houses, their friends. In an instant, everything's gone," Norma Bautista, whose son survived the tornado, told CNN.

President Barack Obama has declared the Oklahoma devastation a major disaster and directed federal authorities to join the rescue and recovery operation. Obama spoke to Oklahoma Governor Mary Fallin by phone and expressed concern, according to the White House.

"Our worst fears are becoming realised this afternoon. We certainly hope everyone heeded the warnings, but it's a populated area and we just fear that not everyone may have gotten the word," said Bill Bunting of the National Oceanic and Atmospheric Administration's Storm Prediction Centre.

Weather officials had issued a storm warning 16 minutes ahead of the touchdown, which is slightly better than the average eight-to-10 minute warning.

The National Weather Service ranked the tornado in the second most powerful category of EF4 on the Enhanced Fujita Scale.

In May 1999, the same town was struck by a severe tornado killing more than 40 people.