Cyber attacks are escalating from large-scale theft and disruption of computer operations to more lethal attacks that destroy systems and physical equipment, according to the head of the US Cyber Command.
"That's our concern about what's coming in cyberspace - a destructive element," General Keith Alexander told a US conference on cyber warfare, according to the Washington Times.
Alexander, who is also the director of the National Security Agency (NSA), said that future computer-based combat is likely to involve cyber strikes that cause widespread power outages and even physical destruction of machinery.
The potential for cyber attacks to do this, he said, is illustrated by the electrical power outage in the Northeast US in 2003 caused by the freezing of software that controlled the power grid after a tree damaged two high-voltage power lines, and the destruction of a water-driven electrical generator at Russia's Sayano-Shushenskaya dam in 2009 that was caused by a computer operator remotely starting the generator while one of the dam's turbines was being serviced.
These events highlight the threat of attackers breaking into electricity grid networks or remotely starting or stopping systems to cause destruction and loss of life, said Alexander.
The US government is adopting an "active defence" strategy aimed at bolstering the readiness of computer networks to respond.
The UK government has come under fire from the Chatham House think-tank for failing to take a strong lead in protecting critical systems such as power and water from cyber attack.
There is no coherent picture of who is targeting what and which systems and services are potentially vulnerable to cyber attack, according to a Chatham House report.
The UK government must play "an integral role in informing wider society" and raising levels of awareness, said the report, which is based on a series of interviews with senior figures in companies considered to be part of the critical national infrastructure, such as electricity, oil and gas.
The Chatham House report comes ahead of the government's expected announcement of a revised cyber security plan.