Criminals have been able to hack into computer systems and cut power to several cities, according to a CIA analyst.

Criminals have launched online attacks that disrupted power equipment in several regions outside of the US, said Tom Donahoe, without identifying the countries affected. The goal of the attacks was extortion, he said.

"We have information, from multiple regions outside the United States, of cyber intrusions into utilities, followed by extortion demands," he said, speaking at a computer security conference. "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." He posted a statement on the website of the SANS Institute, the organiser of the conference.

"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," said SANS.

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.

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.