Cybersecurity experts from around the world meeting on ways to protect the Internet say they still have fears of "nightmare" scenarios in which attacks could cripple critical computer networks.

"I live in a world of nightmares," Patrick Pailloux, director general of France's Network and Information Security Agency, told participants in the first Worldwide Cybersecurity Summit which ended on Wednesday.

"Each subject is a nightmare: electricity, power grids, transportation, airplanes, water supply, finance, the banking system, the health system," Pailloux said.

"My biggest nightmare is that we don't have enough time to prepare us for the nightmares," said the head of France's cyber defense efforts.

Pailloux was among the 400 participants from 40 nations who attended the meeting hosted by the EastWest Institute think tank to come up with ways to protect the world's digital infrastructure from cyber threats.

The cybersecurity experts, government officials and business leaders agreed that only global cooperation could protect computer networks under constant attack from ever mutating viruses, worms, spam and a host of other dangers.

White House cybersecurity coordinator Howard Schmidt said his "nightmare scenario" would involve a loss of communications.

"You have some kind of a physical event -- whether it's a natural disaster, whether it's a direct attack -- and you somehow because of a cyber insecurity do not have an ability to communicate or direct emergency response," he said.

"As far as a single incident that could take place that's probably the one I worry about the most," Schmidt said.

Michael McCaul, a Republican member of the US House of Representatives from Texas, expressed concern about a "denial of service attack that shuts down power grids and causes major blackouts across the country."

Denial of service attacks can paralyse websites by overwhelming them with requests from thousands of zombie computers.

"Everything is tied to the Internet therefore everything is vulnerable," McCaul said. "With a click of a mouse you can blow up power grids."

"The energy sector is really the Achilles heel of every nation and every business and every citizen," said Melissa Hathaway, the former acting senior director for cyberspace for the US National Security and Homeland Security Councils.

McCaul also said not all threats exist in cyberspace, pointing to the undersea cables that carry Internet traffic between the continents.

"The Internet cable is a physical thing," he said. "It's not virtual.

"It's exposed off the coast of Egypt. That could be physically attacked and would literally shut down the Internet," he said.

A number of participants in the conference said one of their greatest fears is that people could simply lose confidence in the Internet unless measures are taken to crack down on such things as identify theft and credit card fraud.

If the public decides over time that the Web is no longer a safe place to do business it could deal a crippling blow to electronic commerce, they warned.

"If the bad guys win they erode the fundamental confidence that people have in communications networks and services," said Randall Stephenson, chairman, chief executive and president of US telecom giant AT&T.

"And I tell you that confidence is critical," he said in a closing speech to the gathering. "Without it all this great human and economic progress that's been powered by advanced communications stands at risk."