The Three Mile Island nuclear power plant in Pennsylvania is on high alert after receiving a credible threat against the installation, plant officials said.
"We were notified last night that a security threat had been made against Three Mile Island. That threat was deemed credible. We took extra security measures and we remain at that heightened state of alert," said Mr David Carl, spokesman for operators Exelon Nuclear.
Nearby Harrisburg International Airport and Lancaster airport were shut down for four hours because of the threat, which came a week after the FBI issued a warning about possible additional attacks on US interests at home or abroad following the September 11th assaults on New York and Washington by hijacked commercial airliners.
In 1979, Three Mile Island was the site of the worst nuclear accident in the United States. There have concerns expressed in Ireland that Sellafield nuclear plant in Cumbria could be a target of an attack.
The Exelon Nuclear spokesman, said work continued overnight at the plant and that none of the 800 or so workers was evacuated. He declined to give details of the specific nature of the threat, but said Three Mile Island had activated its emergency response facilities, including a technical support center and information center, as a precaution.
"We did not reach a level where we had to declare a formal emergency," Mr Carl said. He said no incident had taken place overnight but that heightened security measures would remain in place for as long as needed. The FBI and Federal Aviation Administration could not be reached for comment.