Cyber Storm took place on computers isolated from the net

IT companies and state and foreign governments also played a role in responding to the mock attacks.

The exercise had given the US "an excellent opportunity to enhance our nation's cyber security," the US said.

"Cyber security is critical to protecting our nation's infrastructure," George Foresman of the Department of Homeland Security added in a statement.

The US has been accused of being unprepared for a determined attack by hackers.

Cyber Storm reportedly not only tested against attacks by hackers, but also by bloggers - who deliberately spread misinformation in the exercise.

It was carried out on secure computers in the basement of the Secret Service in Washington DC.

There was no effect on the internet.