Shamoon virus "most destructive yet"

Addressing business leaders in New York, Panetta described the virus as sophisticated and noted that a similar attack days later struck Qatar's natural gas firm, Rasgas.

"More than 30,000 computers that it infected (at ARAMCO) were rendered useless, and had to be replaced," he said.

He said Shamoon included a routine called a "wiper," coded to self-execute, which replaced crucial system files with an image of a burning U.S. flag. It also overwrote all the real data on the machine with what he called garbage data.

"Imagine the impact an attack like this would have on your company," Panetta said, as he called for steps to bolster the nation's cyber defenses.

In his speech, Panetta also cited recent denial-of-service attacks against major U.S. banks, which delayed or disrupted services on customer websites.

"While this kind of tactic isn't new, the scale and speed with which it happened was unprecedented," he said.

One U.S. official, briefing reporters before the speech on condition of anonymity, said the United States knew who carried out the attacks cited in Panetta's speech, but declined to disclose that information.

Copyright Thomson Reuters 2012