The US Nuclear Regulatory Commission has warned that the nation's 103 nuclear power plants could be a target of an airline attack, US officials said on Thursday.

"The NRC issued a message (to nuclear operators) saying you need to be aware that there is some information that indicates a nuclear power plant could be a target and to be aware of your surroundings and report anything unusual to the FBI," said Gordon Johndroe, spokesman for Homeland Security Director Tom Ridge.

Another US official said the NRC document was dated Jan 23 and it cited information passed on by the FBI after a senior al-Qaeda operative was debriefed. Osama bin Laden and his al-Qaeda network have been blamed for the Sept 11 hijacked plane attacks on the World Trade Center and the Pentagon.

The document warned of a second airline attack on America, this time on a nuclear power plant, the official said.

The information was uncorroborated and had no specific date or time for any attack, Johndroe said.

He noted that President George W Bush said in the State of the Union address on Tuesday that al-Qaeda was gathering information on potential targets inside the United States.

"This is why we remain on alert," he said.

In his speech on Tuesday Bush said documents found in Afghanistan showed that the war against terror was just beginning.

"We have found diagrams of American nuclear power plants and public water facilities, detailed instructions for making chemical weapons, surveillance maps of American cities, and thorough descriptions of landmarks in America and throughout the world," Bush said.

An FBI spokesman said the bureau had not issued any public warning about an attack on a nuclear power plant.

NRC spokesman Roger Hannah would not comment on the docu-

ment but said: "Up to this point, there has been no credible threat to any nuclear power plant in this country."

Hannah said security at US nuclear power plants remained "on heightened alert" since the Sept 11 attacks. He said there has been no changes this month in that security level.

The NRC warning comes on the heels of media reports that US intelligence agencies have issued an internal alert warning that Muslim extremists were planning to strike again, possibly targeting a US nuclear power plant.

The NRC document reportedly cited information from the FBI that said a second airline attack on the United States was already planned and three individuals were on the ground in the United States recruiting non-Arabs to take part.

It said the plan was to fly a commercial aircraft into a nuclear power plant, and if any military aircraft intercepted the plane the mission would be diverted to any tall building.