Worst hit was Moore, south of the city, where neighbourhoods were flattened² and schools destroyed by winds of up to 200mph (320km/h).

 

About 120 people are being treated in hospitals.

 

President Barack Obama has declared a major disaster in Oklahoma.

 

He also ordered federal authorities to join in the search efforts which are continuing throughout the night.

 

Monday's twister hit Moore, a suburb of about 55,000 people, at 14:56 (19:56 GMT) and remained on the ground for about 45 minutes.

 

The official death toll³ is 51, but local authorities say the figure is expected to rise as another 40 bodies have been found. 

 

The Oklahoma chief medical examiner's office said at least 20 children were among the dead. 

 

Plaza⁴ Towers Elementary school took a direct hit: the storm tore off the building's roof and knocked down walls.

 

"The school was flattened. The walls were pancaked in," Oklahoma's Lt Gov Todd Lamb told the BBC. 

 

"There's still roughly two dozen children that are missing. There have been some bodies recovered from that school and it's absolutely horrific and devastating⁵."

 

Another school - Briarwood Elementary - was also damaged, and teachers were later seen leading pupils out to safety. 

 

Oklahoma Gov Mary Fallin said it was a "tragic⁶" day.

 

More than 200 Oklahoma National Guardsmen as well as out-of-state personnel have been called in to assist the search-and-rescue effort.

 

The BBC's Alastair Leithead in Oklahoma says many of tornadoes⁷ in the region hit the open plain, but this one struck a residential⁸ area.

 

Many houses are built on hard ground without basements, so many people did not have recourse to shelter, our correspondent adds.

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