The report in the Wall Street Journal quotes intelligence officials saying that cyber-spies last year repeatedly gained access to the system powering everything from financial institutions to sewage systems.

'The Chinese have attempted to map our infrastructure, such as the electrical grid,' the daily said quoting an unidentified senior intelligence official, and 'so have the Russians.'

While no damage was caused, investigators found time bomb style viruses sown into the system. 'If we go to war with them, they will try to turn them on,' the official was quoted as saying.

Amy Kudwa, spokeswoman for the Department of Homeland Security, said she was 'not aware of any disruption to the power grid.'

However, she acknowledged that 'cyber attacks are made all the time.'

President Barack Obama, the most Internet-savvy president in US history, has pledged to expand the use of the Internet across the country.

Among the projects is a major overhaul, with increasing reliance on computer networks, of the electric grid.

That adds to pressure to defend against the growing army of cyber assailants.

On Tuesday the Pentagon revealed that more than US$100 million (S$150 million) had been spent just in the last six months to repair damage from attacks. -- AFP