THE computer networks that control power plants and financial systems will never be secure enough, so government and corporate leaders should consider developing a new, highly secure alternative internet, a top FBI official said today.

Executive Assistant Director of the FBI Shawn Henry said critical systems are under increasing threat from terror groups looking to buy or lease the computer skills and malware needed to launch a cyber attack.

Mr Henry said jihadist militants looking to harm the U.S. can tap organized crime groups who are willing to sell their services and abilities to attack computer systems.

He would not say which terror group or whether any insurgent networks have actually been able to acquire the high-tech capabilities.

But he said one way to protect critical utility and financial systems would be to set up a separate, highly secure internet.

Mr Henry said cyber threats will always continue to evolve and outpace efforts to defend networks against them, at a conference of the International Systems Security Association.

"We can't tech our way out of the cyber threat," Mr Henry said. "The challenge with the internet is you don't know who's launching the attack."

A key step, he said, would be to develop networks where anonymity is not an option and only known and trusted employees have access.

The vulnerabilities of critical systems such as power plants, the electric grid or Wall Street were a prime topic during the conference, reflecting growing concerns by U.S. officials.

Government security officials say cyber attackers are using the Internet to steal money, ferret out classified secrets and technology and disturb or destroy important infrastructure, from the electrical grid and telecommunications networks to nuclear power plants and transportation systems.

The head of the Pentagon's Cyber Command General Keith Alexander warned that the attacks against critical systems are increasingly carrying destructive viruses or malware.

General Alexander - who also is Director of the US’s National Security Agency - said the Pentagon and intelligence agencies must do more to protect their computer systems and coordinate with private companies to safeguard public networks.

And when a computer network is infected, someone should be able to disconnect it, he said.

"Is it the FBI? Is it the NSA? Is it the military or is it the ISPs - the Internet Service Providers? But somebody can turn that device off," General Alexander said.

He added that the Defense Department is finalizing policies that will determine what the military can do in the event of a cyber attack.

The Defense Department has set up a trial program to share cyber threat data with some large military contractors in order to prevent intrusions.

The Homeland Security Department is looking at that model to protect power plants, financial networks or other key systems.

He also said defense and intelligence agencies will move to cloud computing, which would use highly secure, encrypted banks of remote computers to store data - much like people store photos or email in popular online programs.

Doing that, said Alexander, will allow officials to better see and block any threats trying to get into government systems.