Defense Secretary Leon E. Panetta warned Wednesday that a cyberattack on the United States could cause “another Pearl Harbor” by blacking out private and government electric power grids and throwing the nation into a panic.

“I think the more this technology develops, the more the will to potentially use it is going to take place,” he testified before the Senate Appropriations subcommittee on defense. “I think there’s a high risk.”

Mr. Panetta said U.S. private and public sectors are targets of hundreds of thousands of attacks every day.

“I’m very concerned at the potential in [cyberwarfare] to be able to cripple our power grid, to be able to cripple our government systems, to be able to cripple our financial systems,” he said.

“It would virtually paralyze this country. And as far as I’m concerned, that represents the potential for another Pearl Harbor, as far as the kind of attack that we could be the target of, using [cyberwarfare].”

Army Gen. Martin E. Dempsey, chairman of the Joint Chiefs of Staff, added that the Defense Department needs to develop rules of engagement for cyberspace.

“These things occur at network speed,” he told the committee. “This is not something where we can afford to, you know, convene a study after someone has knocked out the East Coast power grid.”

Mr. Panetta and Gen. Dempsey also warned lawmakers that the looming “sequestration” defense cuts would be disastrous for the nation’s military preparedness.

The automatic cuts, scheduled to go into effect Jan. 2 if Democrats and Republicans fail to come up with a long-term budget solution, would cost the Pentagon about $500 billion in projected spending over 10 years on top of the $492 billion that President Obama and congressional Republicans already agreed to in last summer’s deficit-cutting budget.

• This article is based in part on wire service reports.

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