US electricity grid hit by cyber attacks: report

Quoting unidentified intelligence sources and homeland security officials, the Wall Street Journal said cyber-spies penetrated the system repeatedly last year, without disrupting it.

"The Chinese have attempted to map our infrastructure, such as the electrical grid," the paper said quoting a senior intelligence official, and "so have the Russians."

Officials fear bugs have been sown in the system and could be used to disrupt the vital networks at a time of war or crisis.

Cybersecurity is seen by the US government as a major vulnerability, with Homeland Security Secretary Janet Napolitano naming it as one of at least 12 areas up for rapid policy review since she took office in January.

There are increasing fears that hackers could hijack computer systems which run critical infrastructure facilities, such as power plants, controlling them remotely and causing chaos inside the United States.