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WASHINGTON (CNN) -- The Department of Homeland Security has sent an advisory to the National Football League and local officials advising of a possible, uncorroborated bomb threat against some NFL stadiums.

The threat, posted on a Web site, alleges that dirty bombs could be used this weekend against seven stadiums -- in Miami, New York, Atlanta, Seattle, Houston, Oakland and Cleveland. The bombs were to be delivered by truck, the posting said.

On its Web site, the U.S. Nuclear Regulator Commission describes a dirty bomb as a device that combines a conventional explosive, such as dynamite, with radioactive material. According to the NRC, a dirty bomb is in no way similar to a nuclear weapon. The presumed purpose of its use would be as a weapon of mass disruption rather than a weapon of mass destruction.

Bomb threat not credible
DHS said the posting is not considered credible and the information was being shared only in an abundance of caution so that league officials and others could determine what steps they want to take.

Homeland Security officials said people should still go about their plans and not avoid stadiums.

Greg Aiello, a spokesman for the NFL, said the league was notified of the uncorroborated threat on Tuesday and that all the teams are aware of it.

"We have been given no reason by the government to believe there is a credible threat," Aiello said. "Our stadiums are very well protected through the comprehensive security procedures we have in place," including bag searches, pat-downs and secure facility perimeters.

Teams respond to threat
Ryan Reichert, director of security for the Houston Texans, told CNN that he was advised of the threat Tuesday evening by the league office.

"Our NFL rep is in contact with the local FBI office in Houston trying to determine the validity of the source," Reichert said. "We are waiting to hear back from them as to what level this is -- whether it's a credible source or some random posting on a Web site. We are kind of in a stand-by mode."

Reichert added that for the time being, no additional security measures are being planned but team officials are keeping "eyes and ears open."

Officials at Tampa's Raymond James Stadium in Florida -- not among those listed on the Web site -- said they were putting extra security around the stadium for the game.

Also in Florida, George Torres, spokesman for the Miami Dolphins, issued a statement.

"Safety is of paramount significance for us at every event," Torres' statement said. "The Department of Homeland Security has judged that this threat is not credible. As we do for every NFL event, we are working with the FBI and NFL security to ensure Dolphin Stadium is safe and secure for our guests and staff."

Officials for the Seattle Seahawks in Washington, New York Giants and New York Jets referred media calls to the league for comment.

The Giants have an away game this weekend, but the Jets will be playing at Giants Stadium in East Rutherford, New Jersey. The Seahawks will be playing at Qwest Field, where officials also refused to comment, referring all calls to the league.

Paul J. Browne, deputy commissioner of police in New York, said the Police Department was aware of the threat "and our counterterrorism posture remains unchanged."

The Cleveland Browns issued a statement saying the Ohio team's officials had met with federal and local authorities.

"We take the safety of our fans very seriously and will remain vigilant with all of our security procedures," the statement said. "We would like to emphasize that our fans should continue to feel comfortable attending Cleveland Browns Stadium."

In Oakland, California, Raiders spokesman Mike Taylor referred callers to the NFL.

"We work closely with a number of government agencies, including the FBI, on an ongoing basis," he said.

Katy Pando, a spokeswoman for the Georgia Dome in Atlanta -- the only indoor stadium mentioned in the threat -- joined others in noting DHS "does not consider this to be a credible threat."

"Safety and security is always a top priority at the Georgia Dome," she said. "We work in coordination with local, state and federal officials -- including Homeland Security -- to ensure we provide a safe game day experience for all of our events."

She noted that all NFL stadiums, including the Atlanta facility, "are very well protected through comprehensive security procedures," but she would not discuss any specific procedures "for obvious reasons."

The Atlanta Falcons issued a similar statement.

The threat against the football stadiums is similar to one last spring that indicated NCAA basketball tournament games would be targeted. DHS issued an advisory about the threat at the time, also saying it was uncorroborated. No attacks materialized.

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