U.S. Defense Secretary Leon Panetta says recent computer virus attacks on Persian Gulf oil and gas companies were “probably the most destructive cyber assault the private sector has seen.”

Panetta says the so-called “Shamoon” virus rendered more than 30,000 computers useless when it hit the Saudi Arabian state oil company, ARAMCO. In some cases, the virus replaced computer files with an image of a burning U.S. flag.

The defense secretary says just days after the incident occurred two months ago, there was a similar attack on RasGas, Qatar's second biggest producer of liquefied natural gas.

Panetta commented on the attacks during a speech to business executives late Thursday as he appealed to them to support cyber security legislation that has stalled in Congress.

He told the group the U.S. was working to aggressively fight such attacks, which have the potential to cause widespread U.S. damage.

“The most destructive scenarios involve cyber actors launching several attacks on our critical infrastructure at one time in combination with a physical attack on our country. Attackers could also seek to disable or degrade critical military systems and communications networks. The collective results of these kinds of attacks could be a cyber Pearl Harbor.''

Panetta did not say what nation he suspected was behind the sophisticated Shamoon virus. However, he did say that Iran had “undertaken a concerted effort to use cyberspace to its advantage.”

He also said Russia and China have advanced cyber capabilities.

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