The US Central Intelligence Agency (CIA) has revealed that hackers have launched online attacks that disrupted power equipment and cut power to several cities outside of the US. Speaking at a conference of security professionals last week, CIA analyst Tom Donahue disclosed the recently declassified attacks while offering few specifics on what actually went wrong.

"We have information, from multiple regions outside the United States, of cyber intrusions into utilities, followed by extortion demands," he said in a statement posted to the web by the conference's organisers, the SANS Institute. "In at least one case, the disruption caused a power outage affecting multiple cities. We do not know who executed these attacks or why, but all involved intrusions through the internet."

"According to Mr. Donahue, the CIA actively and thoroughly considered the benefits and risks of making this information public, and came down on the side of disclosure," SANS said.

One conference attendee said the disclosure came as news to many of the government and industry security professionals in attendance. "It appeared that there were a lot of people who didn't know this already," said the attendee, who asked not to be identified because he is not authorised to speak with the press.

He confirmed SANS' report of the talk. "There were apparently a couple of incidents where extortionists cut off power to several cities using some sort of attack on the power grid, and it does not appear to be a physical attack," he said.

Hacking the power grid made front-page headlines in September when CNN aired a video showing an Idaho National Laboratory demonstration of a software attack on the computer system used to control a power generator. In the demonstration, the smoking generator was rendered inoperable.

The US is taking steps to lock down the computers that manage its power systems, however.

On Thursday, the Federal Energy Regulatory Commission (FERC) approved new mandatory standards designed to improve cybersecurity.

CIA representatives could not be reached immediately for comment.