The head of the powerful National Security Agency, General Keith Alexander, said Monday the US must adopt a law to protect the country from cyberattacks while insisting that it would respect privacy.

Faced with ever-increasing Internet traffic and the more sophisticated nature of cyberattacks, Alexander said passing such legislation was "vital."

"What I think we really need to be concerned about is when these (attacks) transition from disruptive to destructive," Alexander said in a speech at the American Enterprise Institute, a Washington think tank.

"The probability for crisis is mounting."

Alexander said any proposed legislation should ensure that the private sector, especially key infrastructure facilities, inform the government of attacks in real time.

The House of Representatives in April passed legislation protecting US businesses and agencies from cyber-attacks, defying a veto threat from the White House, but the Democrat-led Senate never took up the measure.

Critics say such a law would erode civil liberties by allowing firms to onpass private data.

Alexander, who also heads the US Cyber Command, sought to calm fears about privacy breaches.

"For situational awareness, we need to be able to see what's going on. I don't mean the government has to be in the network to see," he said.

"Like the police force, like the fire department, they don't see around buildings waiting for a fire to come on, you call them when it happens. In cyberspace, I see very much the same thing in our partnership with industry."

There is no need for the government to read personal emails, he said, assuring the audience that the NSA doesn't "hold data on American citizens."