A powerful tornado has struck the outskirts of Oklahoma City in the US, hitting at least two schools and destroying countless homes, according to local news reports.

US media report that at least six people have been killed in Oklahoma City.

Hardest hit on Monday appeared to be Moore, a suburb of 55,000 to the south of the city centre.

Local media reports that so far, four victims - three adults and one infant - have been pulled from the rubble.

An unknown number of children were feared trapped inside the Plaza Towers school in Moore, where anxious parents were being kept at a distance while search and rescue workers scrambled to free the pupils.

School had already been let out for the day due to tornado warnings.

The National Weather Service urged residents to immediately take cover as the storm system in the middle of the country threatened to hit as many as 10 states.

"The tornado on the ground right now is huge and has hit through populated areas," Mary Fallin, Oklahoma governor, said on CNN.

She said it was too early to know the extent of the damage, but local television station KFOR, from its news helicopter, showed live images of widespread debris.

"We anticipate that these storms are going to continue to build around Oklahoma," Fallin said.

Tornadoes are most likely to develop late in the day or in the first half of the night because they develop from thunderstorms and this is their most popular time of day as well. Thunderstorms require a lot of energy to get going. They need the heat of the day to provide this energy, but at the same time the top of the cloud must drop to around minus 40C before lightning develops. This means that it’s often late in the day, or in the first half of the night that thunderstorms to reach their peak, and therefore tornadoes as well. Obviously tornadoes that strike during the night have that added problem that you often cannot see them coming, or may be asleep so don’t hear the warnings. This is when they are at their most dangerous. - Steff Gaulter, Al Jazeera's Senior Weather Presenter

Earlier, Brynn Kerr, a National Weather Service meteorologist, said a tornado warning had been issued for two counties in central Oklahoma.

A warning means that residents should immediately find shelter.

Two people were killed on Sunday from tornadoes in Oklahoma and at least 39 others were injured.

The National Weather Service predicted a 10 percent chance of tornadoes in parts of Texas, Oklahoma, Arkansas, Kansas, Missouri and Illinois.

It said parts of four other states - Wisconsin, Indiana, Michigan and Iowa - have a 5 percent risk of tornadoes.

The area at greatest risk includes Joplin, Missouri, which on Wednesday will mark two years since a massive tornado killed 161 people.

The latest tornado in Oklahoma came as the state was still recovering from a strong storm on Sunday with fist-sized hail, blinding rain and tornadoes.

Two men in their 70s died in the storm, including one at a mobile home park on the edge of the community of Bethel Acres near Oklahoma City, according to Keli Cain, a spokeswoman for the Oklahoma Office of Emergency Management.

Thirty-nine people were injured around the state as storms toppled trees and tore up rooftops, she said.

Several hundred homes and buildings were thought to have been damaged or destroyed and approximately 7,000 customers were left without power in Oklahoma.

"There is definitely quite a bit of damage," Cain said.

Fallin declared 16 counties disaster areas, and she and other local and state officials were touring damaged areas on Monday morning.

More than two dozen tornadoes were spotted in Iowa, Oklahoma, Kansas and Illinois, according to the National Oceanic and Atmospheric Administration (NOAA) and local news reports.

Hail stones, some as large as baseballs, were reported from Georgia to Minnesota, NOAA said.

Power lines downed

Wind gusts of 320kph were measured at Moore, said the US National Weather Service, and speeds of 115kph were reported near Gardner, Kansas, and 90kph in Atchison, Kansas. The high winds toppled trees, downing power lines and smashing cars and rooftops in communities around the Midwest.

A tornado that struck southwest of Wichita, Kansas, on Sunday afternoon was rated an EF1 on Monday by the National Weather Service. The most powerful is an EF5.

The tornado damaged homes and outbuildings, felled trees and knocked out power to about 11,000 residents but caused no injuries, said Sharon Watson, spokeswoman for the Kansas Division of Emergency Management.

"We came through this one very fortunate," Watson said.

In southwest Missouri, a tornado touched down shortly after midnight Monday in Barton County, said Tom Ryan, the county's director of emergency management. The tornado damaged some farm buildings and two houses but caused no injuries, he said, noting that it struck in a rural area.

The storm prompted an unusually blunt warning from the central region of the National Weather Service, which covers 14 states.

"You could be killed if not underground or in a tornado shelter," it said. "Complete destruction of neighborhoods,

businesses and vehicles will occur. Flying debris will be deadly to people and animals."

The tornado season in the US had been unusually quiet until last week, when a tornado struck the town of Granbury, Texas, killing six people.