A US adversary would currently be unable to bring down the entire US electrical grid using cyber weapons but such a scenario is conceivable within two to five years, the former vice chairman of the Joint Chiefs of Staff said Tuesday.

"Today, the likelihood that a nation-state or any actor is going to knock down the entire electrical grid of a country, of the United States let's say, is very remote," retired General James Cartwright said.

"That's probably two to five years off," Cartwright said during a panel discussion at the Washington-based Center for Strategic and International Studies here.

"It's a very difficult thing to do," he said of disabling the entire US electrical grid. "It's not some 19-year-old sitting at a keyboard."

Cartwright, who retired as a four-star general from the Marine Corps earlier this year and now holds the Harold Brown chair in defense policy studies at CSIS, said cyber weapons can potentially have devastating consequences.

"A nuclear weapon could knock out the electrical grid in any city," he said. "Cyber could knock it out in the entire country -- in milliseconds.

"And so that's the worry, that it could progress in that direction," he said. "And now you really are talking about an equivalency of a weapon of mass destruction. And how do we handle that? What do we do about that?"

"You can see a direction in which this threat could move," he said.

Cartwright also said that while the United States possesses cyber warfare capabilities they would probably be employed only as a "supporting arm" in the event of a conventional conflict.

"It's really more in the venue of a supporting arm like artillery or something like that," he said.