BBC.
The US President spelled out the scale of the threat in an opinion piece for the Wall Street Journal.
Foreign governments, crime gangs and individuals were probing America's net defences every day.
The US had to do more to put essential defences in place to avoid the debilitating effects of an attack.
"It would be the height of irresponsibility to leave a
digital backdoor wide open to our cyber-adversaries," he wrote in the
newspaper's editorial column.
Enemies struggling to defeat the US on the terrestrial battlefield may move the conflict to cyberspace, he warned.
Mr Obama rehearsed the potential consequences of a successful
cyber-attack saying it could trigger a financial crisis if banks were
hit. Health emergencies could be caused by infiltrating the computer
systems in hospitals or water treatment plants. And, he said, taking out
power plants could bring entire regions to a standstill.
