Defense Secretary Leon E. Panetta warned Thursday that the United States was facing the possibility of a “cyber-Pearl Harbor” and was increasingly vulnerable to foreign computer hackers who could dismantle the nation’s power grid, transportation system, financial networks and government, The New York Times reports.

He said he was reacting to increasing aggressiveness and technological advances by the nation’s adversaries, which officials identified as China, Russia, Iran and militant groups.

“An aggressor nation or extremist group could use these kinds of cyber tools to gain control of critical switches,” Mr. Panetta said. “They could derail passenger trains, or even more dangerous, derail passenger trains loaded with lethal chemicals. They could contaminate the water supply in major cities, or shut down the power grid across large parts of the country.”

Defense officials insisted that Mr. Panetta’s words were not hyperbole, and that he was responding to a recent wave of cyberattacks on large American financial institutions. He also cited an attack in August on the state oil company Saudi Aramco, which infected and made useless more than 30,000 computers.

Mr. Panetta was also pushing for legislation on Capitol Hill. It would require new standards at critical private-sector infrastructure facilities — like power plants, water treatment facilities and gas pipelines — where a computer breach could cause significant casualties or economic damage.

In August, a cybersecurity bill that had been one of the administration’s national security priorities was blocked by a group of Republicans, led by Senator John McCain of Arizona, who took the side of the U.S. Chamber of Commerce and said it would be too burdensome for corporations.

The most destructive possibilities, Mr. Panetta said, involve “cyber-actors launching several attacks on our critical infrastructure at one time, in combination with a physical attack.” He described the collective result as a “cyber-Pearl Harbor that would cause physical destruction and the loss of life, an attack that would paralyze and shock the nation and create a profound new sense of vulnerability.”

Panetta indicated that the United States might redefine defense in cyberspace as requiring the capacity to reach forward over computer networks if an attack was detected or anticipated, and take pre-emptive action. These same offensive measures also could be used in a punishing retaliation for a first-strike cyberattack on an American target, senior officials said.

This is a summary. See The New York Times article for the complete text. — Ed.