Several children have been pulled out of the rubble alive at a school in an Oklahoma City suburb hit by a tornado.

The children were pulled out of what was left of the school in Moore, Oklahoma after a massive tornado hit the region.

Rescue workers lifted children from the rubble before they were passed down a human chain and taken to a triage centre set up in the school's car park.

The school is southwest of Oklahoma City. Its roof appears mangled and the walls had fallen in or had collapsed.

The National Weather Service said the tornado's preliminary classification was an EF-4, with winds up to 320km/hr.

tornado in the city of Shawnee was one of several that touched down in the US yesterday.

Twisters, hail and high winds also struck Iowa and Kansas as part of a massive, northeastward-moving storm system that stretched from Texas to Minnesota.

Governor Mary Fallin declared an emergency for 16 Oklahoma counties that suffered from severe storms and flooding during the weekend.

The declaration lets local governments acquire goods quickly to respond to their residents' needs and puts the state in line for federal help if it becomes necessary.

Forecasters had been warning of a general storm outbreak since Wednesday.

Tornado watches and warnings were in effect yesterday in much of the nation's midsection.

The National Weather Service office in Norman, Oklahoma issued an insistent alert on Twitter about a tornado striking Pink, a town on the edge of Oklahoma City.

The storm was so large and severe it prompted unusually blunt warnings.

"You could be killed if not underground or in a tornado shelter," the advisory read.

"Complete destruction of neighbourhoods businesses and vehicles will occur. Flying debris will be deadly to people and animals."