A huge tornado flattened an area near Oklahoma City on Monday, tearing up at least two schools and leaving a wake of tangled wreckage as a dangerous storm system threatened as many as 10 U.S. states.

Television video showed homes destroyed, cars tossed and at least one building on fire. Rescue workers were pulling third-graders from a damaged school in Moore, Oklahoma, a KFOR television reporter said from the scene.

"I have never seen anything like this in my 18 years covering tornadoes here in Oklahoma City. This is without question the most horrific," said Lance West, a reporter for KFOR television.

There were no reports of deaths and the number of injuries remained unconfirmed after the tornado struck near Moore, in the Oklahoma City metropolitan area, in midafternoon.

"It seems that our worst fears have happened today," said Bill Bunting, National Weather Service meteorologist in Norman, Oklahoma.

The massive twister struck at the height of tornado season, and more were forecast. On Sunday, tornadoes killed two people and injured 39 in Oklahoma.

Witnesses said Monday's tornado appeared more fierce than the giant twister that was among the dozens that tore up the region on May 3, 1999, killing more than 40 people and destroying thousands of homes.

That tornado ranked as an EF5, the highest on the Enhanced Fujita Scale, meaning it had winds over 200 mph.

The 1999 event ranks as the third-costliest tornado in U.S. history, having caused more than $1 billion in damage at the time, or more than $1.3 billion when adjusted to today's dollars. Only the devastating Joplin and Tuscaloosa tornadoes in 2011 were more costly.

"We have many injured and we're just trying to work out how bad it is right now," said a woman who answered the phone at the Moore city manager's office.

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 and blinding rain.

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, said 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.

Oklahoma Governor Mary Fallin declared 16 counties disaster areas.

MULTIPLE TWISTERS

More than two dozen tornadoes were spotted in Iowa, Oklahoma, Kansas and Illinois, according to the National Oceanic and Atmospheric Administration and local news reports. Hail stones, some as large as baseballs, were reported from Georgia to Minnesota, NOAA said.

Wind gusts of 72 miles per hour (115 kilometers per hour) were reported near Gardner, Kansas, and 60 mph in Atchison, Kansas. The high winds toppled trees, downing power lines and smashing cars and rooftops in communities around the Midwest.

A tornado that touched down southwest of Wichita, Kansas, on Sunday was rated an EF1 on Monday by the National Weather Service. The most powerful is an EF5. The tornado stayed on the ground for about 4.5 miles, with winds of 86-110 mph, the service said.

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.

Just east of Barton County, in Dade County, the tornado tore off roofs at a grocery store, golf course and city swimming pool complex in Lockwood, said Bob Kitsmiller, director of emergency management for the county, adding that no injuries were reported.

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