The US general nominated to lead a new cyber command on Thursday struggled to explain to lawmakers how the United States would wage a digital war, saying key questions remained unanswered.

Lieutenant General Keith Alexander said the new command would operate under the laws of armed conflict but that much about cyber warfare remained "unchartered territory."

Carl Levin, chairman of the Senate Armed Services Committee, presented the general with a number of scenarios at a hearing, including a possible attack on the country's electricity grid staged through a US-based computer network.

Alexander said such a scenario "brings in the real complexity of the problem that we face today, because there are many issues out there on the table that we can extend, many of which are not yet fully answered."

Responding to an attack orchestrated through US networks meant "civil liberties, privacy all come into that equation ...while you try to, on the same network, potentially take care of bad actors."

He said the military would play a supporting role to the Department of Homeland Security and other civilian agencies in the event of an assault on US networks, unless there was a presidential order.

Alexander is already head of the ultra-secret National Security Agency, which carries out vast electronic surveillance operations, including international phone calls to and from the United States.

His nomination to direct the military's first cyber command has raised concerns among rights groups and members of Congress that civil liberties could be placed at risk, with some arguing the task of espionage should be kept separate from waging cyber war.

The general said the cyber command and NSA would work as separate entities, and pledged to uphold legal safeguards for civil liberties and privacy.

In his written testimony submitted in advance, Alexander confirmed that the new cyber command would be prepared to wage offensive operations as well -- despite the risk of sustaining damage to US networks.

"Even with the clear understanding that we could experience damage to our infrastructure, we must be prepared to 'fight through' in the worst case scenario," he wrote.

He told lawmakers that he expected digital operations to take place as part of a wider military campaign, but that special legal authority would be required to respond to a cyber attack staged from a neutral country.

One lawmaker expressed some frustration that Alexander spoke of promoting "transparency" but had declined to answer publicly key questions from the committee, preferring to keep his responses "classified."

"Is there anything you can tell us in this open session to get at some of those basic questions?" Senator Mark Udall asked.

Alexander replied that transparency was "important" but did not offer to lift the lid on his earlier "classified" answers.

The general also said the cyber threat to US networks was growing, with "hundreds of thousands of probes" a day, he said.

The military was "alarmed by the increase, especially this year," he said.

But the probes were not necessarily hostile attacks and could be attempts to scan the government's networks for weaknesses, he said.