(CNN) -- Severe weather's assault on middle America continued Tuesday, as tornadoes and thunderstorms pulverized communities and claimed at least six lives in Oklahoma and Kansas.

Twisters also were brewing in Dallas and several northern Texas counties, according to the National Weather Service, with at least one tornado reported on the ground.

There was no rest for the weary and grieving.

Joplin, Missouri, where 124 died in a tornado Sunday, was under a tornado warning late Tuesday.

Two motorists died when an uprooted tree slammed into their van in Stafford County, Kansas, according to the state adjutant general's office.

The deadly string of tornadoes that rampaged through central Oklahoma killed at least four people, injured dozens and destroyed homes and vehicles, officials said.

Canadian County, Oklahoma, Sheriff Randall Edwards told CNN a large tornado that crossed I-40 near El Reno destroyed residences and caused a gas leak at an energy plant west of the state capital.

Four people died in the county, said Cherokee Ballard, spokesperson for the state medical examiner.

The twister injured motorists on I-40 and U.S. 81, Canadian County Emergency Management Director Jerry Smith said. Deputies tended to the injured, and there were reports of property damage in the area.

El Reno city spokesman Terry Floyd said 20 workers were injured at a drilling rig.

Another tornado was seen at Chickasha, about 40 miles southwest of Oklahoma City. It later reached Newcastle, before pushing through Moore and Norman, suburbs of Oklahoma City. The National Weather Service warned residents and I-44 drivers to take precautionary action.

About 1,200 people packed a shelter in Newcastle, a bedroom community near Oklahoma City, during the storm, said City Manager Nick Nazar. "That saved lives."

"We have been extremely lucky," he said. "Minor injuries so far."

About 100 people were displaced, and 50 homes were rendered uninhabitable, Nazar told CNN. Two or three businesses were damaged, as was an elementary school.

Statewide, at least 60 people were hurt and nearly 58,000 homes lost power, according to the Oklahoma Department of Emergency Management. Damage was reported in at least 14 counties.

The tornado that passed through Chickasha also damaged several other communities, including Newcastle.

"It came right past the store," said Chickasha AutoZone employee Nathaniel Charlton. "They had a little debris thrown across the parking lot. It was on the ground, but it wasn't bad."

Sirens went off about 20 minutes before the storm pushed through, Charlton told CNN.

State officials received reports of damaged businesses in Chickasha.

"This is an extremely dangerous and life-threatening situation," the National Weather Service said during the storms.

No tornado damage was immediately reported within Oklahoma City's borders.

Some employees at the weather agency's Storm Prediction Center in Norman briefly took shelter as a tornado approached, a spokesman told CNN.

Oklahoma Gov. Mary Fallin cited the storms' intensity.

"We're still not out of the woods yet," she told CNN's Anderson Cooper as strong storms moved across her state Tuesday evening. First responders were heading toward communities that have reported damage to homes, Fallin said.

Randy McCown, administrator at First Baptist Church of Piedmont, northwest of Oklahoma City, said the church was operating as a shelter for residents. Heavy damage was reported across the city.

CNN Oklahoma City affiliates broadcast images of funnel clouds that dumped rain as they moved into more populated areas.

KWTV broadcast images of destroyed and damaged homes in El Reno.

The University of Oklahoma, based in Norman, on Tuesday afternoon suspended classes at its three campuses.

At Norman's Tarahumara Restaurant, which specializes in Mexican food, all 20 employees showed up for work, though none of the customers did for nearly two hours preceding the storm's arrival, said manager Juan de Leon.

As the storm struck at about 5:45 p.m., he and the wait staff watched news programs on the restaurant's 10 television sets in fear, he said. But when it hit, they were unimpressed. "Just rain," said de Leon in a telephone interview. "It looked like normal rain." Hail was mixed in, but the bits of ice were nothing special, "little bitty ones," he said.

Two hours later, he said, the restaurant had filled up and it was business as usual.

Tornado watches were in effect Tuesday evening in Texas, Missouri, Arkansas, Oklahoma and Kansas.

In anticipation of the severe weather, American Airlines canceled 126 arriving and departing flights at Dallas-Fort Worth International Airport, spokesman Ed Martelle told CNN.

Operations were suspended late Tuesday afternoon at Will Rogers World Airport in Oklahoma City.

The Storm Prediction Center said the high-risk area for severe storms includes southern Kansas, most of Oklahoma and southward into Texas.

Surrounding the high-risk area is a large "moderate"-risk area where tornadoes are possible. This includes the cities of Dallas; Kansas City, Missouri; Springfield, Missouri; and Joplin.

A tornado was reported Tuesday in the Texas community of Bedford.

The tornado that struck Joplin on Sunday killed at least 124 people, authorities said Tuesday, making it the deadliest single U.S. tornado since modern record-keeping began more than 60 years ago.

CNN's Dave Alsup, Matt Smith, Tom Watkins, Phil Gast, Dana Ford, Sean Morris and Joe Sutton contributed to this report.