Cyber-attacks could inflict as much damage on the US as the physical attacks on 11 September 2001, the US defence secretary has warned.

Leon Panetta said the country was preparing to take pre-emptive action if a serious cyber-attack was imminent.

He said US intelligence showed “foreign actors” were targeting control systems for utilities, industry and transport.

Advanced tools were being created to subvert key computer control systems and wreak havoc, said Mr Panetta.

“An aggressor nation or extremist group could gain control of critical switches and derail passenger trains, or trains loaded with lethal chemicals,” said Mr Panetta in a speech to business leaders held on the USS Intrepid – a former aircraft carrier that is now a museum.

“They could contaminate the water supply in major cities, or shut down the power grid across

Read more: BBC News – US prepares first-strike cyber-forces.