Cyberattacks on US networks by other nations may not always demand the same level of retaliation, and only attacks that cause major damage or loss of life should prompt similar responses, a group of national security experts said on Wednesday.

Cyberattacks on private companies and even on the US Department of Defense's network are commonplace and part of a long history of international espionage that the US and other countries have engaged in for years, said some panellists speaking at a cyberwar discussion at the Center for Strategic and International Studies (CSIS) in Washington, DC.

Presented with a scenario similar to a computer compromise at RSA Security, in which the attackers also targeted defence contractor Lockheed Martin, defense consultant Franklin Miller said people getting upset with the attack seem to assume that the US doesn't engage in some of the same practices.

"This is going to happen," said Miller, former senior director for defense policy and arms control at the US National Security Council. "This is what intelligence organisations do."

CSIS fellow Adriane Lapointe presented Miller and other panelists with a series of reality-based scenarios and asked them what the appropriate US government response should be. The CSIS panel came just days after the US Department of Defense said it was prepared to use force to respond to some cyberattacks.

In some cases, attacks that appear to be sponsored by another nation - including attacks that Google blamed on China in early 2010 - may prompt the US to file formal complaints, the panelists said. In other cases, the US may want to signal another country that it considered an attack unfair, although it may be more difficult for other countries to read cybersignals than to interpret military aircraft flying near their borders, said James Lewis, director of the CSIS Technology and Public Policy Program.

The cybersignals of displeasure can be clear, said Robert Giesler, former director of information operations and strategic studies in the DOD's Office of the Secretary of Defense. "There are ways you can noisily go about penetrating networks," he said.

In some cases, the signal may not be cyber in nature, Miller added. "You don't have to [signal] in the same medium," he said. "If you want to inflict pain... you need to find the point of pain. It may not be in the cyberworld. It may be elsewhere."

Countries may need to negotiate rules of engagement for cyber-espionage, panelists said.

Part of the problem with retaliating against other nations for a cyberattack is that it's still difficult to pinpoint where an attack came from, Giesler said. In some cases, compromised companies may attribute an attack to another nation when it's really a case of industrial espionage, he said. In other cases, attacks may come from so-called "patriotic" hackers who are acting on their own initiative, he said.

Even in attacks on DOD networks, "attribution still remains foggy," Giesler said.

Asked about attacks on DOD networks by another country, the panelists said the US should respond, but in most cases, in a limited way. Only if major damage was done should the U.S. consider responding with force, said Judith Miller, former general counsel at the DOD.

"You're not going to start a war over something like this," added Robert Deitz, former senior councillor to the director of the US Central Intelligence Agency.

The response would change in the case of an attack causing major damage or killing US residents, the panelists said. An attack that takes down a large portion of the US electric grid or the banking system would likely require significant retaliation, Franklin Miller said.

Recent news reports have suggested that China and Russia have probed the US electric grid for weaknesses. Panelists suggested that attacking the grid was a step up from probing it.

"This is the kind of message that needs to be put out by the United States government publically, that inference with the grid constitutes an extremely serious act ... which would be subject to very serious retaliation," Franklin Miller said. "If you lose the ability to generate or distribute power to an entire region of this country, we are going to be in very serious trouble as a country."