Major power plants are one of the possible targets of planes that may be hijacked from Canada, Mexico or the Caribbean, according to a security warning issued by the FBI and Department of Homeland Security.

The warning was sent to government and industry officials late on Friday even though the source of the threat was a single individual of uncertain reliability, the reports said.

"We haven't had the opportunity to fully judge his credibility," an official familiar with the source told Saturday's Washington Post. Nevertheless intelligence officials judged it credible enough to issue a warning, the official said.

Riyadh threats

US authorities were also preparing to send a similar alert to Saudi Arabia, warning that cargo planes could be used as weapons against targets there, CNN reported.

Security standards for cargo planes are not as high as those imposed on passenger planes in the United States in the wake of the 11 September 2001 attacks.

Meanwhile, the US embassy in Riyadh warned on Friday of a serious threat of attacks in Saudi Arabia and announced that its missions there would remain closed for a security review. The embassy was closed down on Saturday without a forecast date for reopening.