Nuclear plant put on high alert after 'credible threat'

A US nuclear power plant is on a high state of alert after receiving a "credible threat".

Airspace around the plant at Three Mile Island has also been closed.

Military aircraft are patrolling and the FBI and state police are watching over the premises.

The power plant has been taking additional security measures after the September 11 terrorist attacks.

A spokesman has confirmed the plant's high state of alert, but has declined to discuss the type of threat or the additional security measures.

Three Mile Island is located just outside Harrisburg.

In 1979, the plant was the site of America's worst commercial nuclear accident.

Around a third of the nuclear fuel melted inside a reactor and radiation leaked into the atmosphere.