At least two tornadoes ripped across northern Texas Tuesday, the National Weather Service said, causing major damage and grounding flights, but leaving no reported casualties.

"Tornado Emergency. Two tornadoes are currently affecting the DFW (Dallas-Fort Worth) Metroplex. Take cover now!" the service's online alert board said.

As residents cowered in shelters, local television images showed school buses, trucks and train cars that had been tossed through the air before plunging to the ground, along with houses damaged by the powerful twisters.

A tornado watch was issued for the Dallas-Fort Worth metropolitan area -- home to more than six million people -- until 8:00 pm (0100 GMT Wednesday).

"911 (emergency services) is going off the wall, with people calling in, sightings of tornadoes, possible sightings of tornado as of up to about 20 minutes ago," Lieutenant Tim Jones of Johnson County Sheriff's Office told CNN.

"Now we get people calling to make sure their relatives are okay."

Texas congressman Michael Burgess, a Republican, told MSNBC he worried that the intensity of the storms could mean significant damage.

"When it passed over, the rain was so intense that you couldn't see much of anything. A lot of lightning and thunder and wind," Burgess said. "Other areas I fear... have probably suffered more greatly."

David Magana, a spokesman for the busy Dallas-Fort Worth airport, told CNN that federal authorities had issued a ground stop, meaning all flights headed to Dallas had been held on the ground at their airports of origin.

Airport passengers were sheltering at the huge facility, though flights were expected to soon resume, Magana said.

American Airlines however was suspending operations for the rest of the day, a spokesman for the airline, Tim Smith, told Fox News.

"I do not know of any significant damage at the airport. We ask folks to please understand and bear with us. We'll get things up and running fast as we can," he said.

Despite widespread damage, there were no immediate reports of casualties.

"It got dark pretty quick. And we've had a couple of, at least two tornadoes, sighted in Dallas and Tarrant county," Richard Hill, Dallas public information officer, told MSNBC.

Residents are "all taking shelter, finding shelters, schools, and universities, in the area," Hill added.

Texas Senator John Cornyn said in a statement that he and his staff would be working to assist all those hit by the storms.

"My thoughts and prayers are with those affected by today's violent weather across north Texas," he said in a statement.